LIBRARY 
CALIFORNIA 

COLLEGE 
OF 


MEDICAL 


COLLEGE  OF  PHARMACY 


AN    INTRODUCTION 

TO 

VEGETABLE   PHYSIOLOGY 


AN  INTRODUCTION 

TO 

VEGETABLE  PHYSIOLOGY- 


BY 

J.  EEYNOLDS  GEEEN,  Sc.D.,  F.E.S.,  F.L.S. 

FELLOW  AND  LECTURER  OF  DOWNING  COLLEGE,  CAMBRIDGE 

HARTLEY  LECTURER  ON   VEGETABLE  PHYSIOLOGY  IN    THE  UNIVERSITY  OF  LIVERPOOL 

LATE  PROFESSOR  OF  BOTANY   TO    THE  PHARMACEUTICAL   SOCIETY  OF  GREAT  BRITAIN 

FORMERLY  SCHOLAR  OF   TRINITY  COLLEGE   AND  SENIOR  DEMONSTRATOR 

IN  PHYSIOLOGY  IN   THE  UNIVERSITY  OF  CAMBRIDGE 


THIRD    EDITION 


PHILADELPHIA 

P.   BLAKISTON'S   SON  &  CO. 

1012   WALNUT    STREET 

1911 


Printed  in    Great  Britain 


PREFACE 

TO 

THE    THIRD    EDITION 

TEN  years'  experience  of  the  use  of  this  book  with  students 
has  led  me  to  make  many  alterations  in  the  details  of 
the  treatment  of  the  subject.  In  a  few  places  consider- 
able expansion  has  seemed  called  for,  particularly  in  the 
presentation  of  the  general  physiology  sketched  in  the 
second  chapter.  Hence  certain  sections  of  the  book 
have  been  re-written.  In  other  places  re-arrangement 
has  proved  advantageous — particularly  in  the  case  of  the 
section  dealing  with  the  energy  of  the  plant.  This  has 
accordingly  been  re-cast,  and  the  chapter  on  respiration 
has  been  incorporated  with  it,  a  change  partly  carried  out 
in  the  second  edition. 

The  past  ten  years  have  seen  many  advances  made  in 
experimental  work  and  in  the  suggestion  of  new  theories. 
I  have  endeavoured  to  incorporate  as  much  of  this  as 
seems  to  me  sound,  and  I  have  eliminated  certain  of  the 
older  statements  which  more  recent  work  has  shown  to  be 
certainly  or  probably  erroneous. 

In  what  I  have  added  I  have  dealt  with  the  correlation 
of  internal  structure  with  physiological  need  to  a  greater 
extent  than  in  the  earlier  editions,  and  have  examined 

424' 


vi  VEGETABLE  PHYSIOLOGY 

more  fully  than  before  the  general  relations  of  the  in- 
dividual and  its  environment. 

I  have  set  myself  throughout  to  combat  an  idea  that 
has  arisen  during  the  last  few  years  that  many  changes 
may  go  on  in  protoplasm  without  involving  any  inter- 
change with  its  substance.  This  I  hold  to  be  erroneous. 
In  all  the  reactions  of  which  it  is  the  scene  its  own  auto- 
decomposition  and  reconstruction  are  involved. 

I  have  sought  to  keep  the  book  within  practically  the 
same  dimensions  as  in  former  editions,  and  have  conse- 
quently compressed  certain  of  the  less  important  parts. 
When  introducing  new  matter  I  have  tried  to  avoid  undue 
speculation  as  tending  to  confuse  an  elementary  student. 

J.  EEYNOLDS  GBEEN. 
CAMBRIDGE,  May  1911. 


PREFACE 

TO 

THE    FIRST    EDITION 

ALTHOUGH  during  recent  years  considerable  additions  have 
been  made  to  our  elementary  botanical  textbooks,  not  one 
has  appeared  which  deals  solely,  or  at  any  length,  with  the 
subject  of  vegetable  physiology.  This  has  been  either 
presented  to  the  reader  as  a  particular  section  in  a  com- 
prehensive work,  or  treated  of  incidentally  in  connection 
with  anatomical  detail.  This  is  the  more  strange,  as  an 
adequate  and  intelligent  appreciation  of  the  forms  and 
structure  of  vegetable  organisms  can  only  be  gained  by  a 
consideration  of  the  work  they  have  to  carry  out.  It  must 
be  evident  to  the  student  of  Nature  that  the  peculiarities 
of  external  and  internal  form,  of  which  any  particular  plant 
has  become  possessed,  have  arisen  necessarily  in  con-. 
nection  with  the  need  of  mechanisms  to  do  certain  work, 
to  overcome  particular  disadvantages,  and  generally  to 
bring  the  organism  into  a  satisfactory  relationship  with  the 
surroundings  among  which  it  finds  itself. 

I  have  been  led  by  these  considerations  to  endeavour  to 
fill  this  gap  by  writing  an  introduction  to  the  subject, 
which,  while  putting  physiology  into  its  proper  prominence 
among  the  branches  of  botanical  study,  shall  serve  to  pave 

the  way  of  the  student  and  of  the  general  reader  to  the 

b 


viii  VEGETABLE  PHYSIOLOGY 

more  complete  discussion  of  the  subject  which  may  be 
met  with  in  the  advanced  textbooks  of  Sachs,  Vines,  and 
Pfeffer. 

With  this  view  I  have  endeavoured  to  present  the 
plant  as  a  living  organism,  endowed  with  particular 
properties  and  powers,  realising  certain  needs,  and  meet- 
ing definite  dangers.  I  have  attempted  to  show  it  to  be 
properly  equipped  to  encounter  such  adverse  conditions, 
and  to  avail  itself  of  all  the  advantages  presented  to  it 
by  its  environment. 

I  have  also  set  before  myself  another  purpose,  which, 
however,  is  naturally  subordinate  to  the  one  just  mentioned. 
When  we  consider  the  origin  of  the  different  organisms 
which  we  find  around  us,  we  are  led  irresistibly  to  the 
conclusion  that  the  classification  of  living  beings  into 
animals  and  plants  has  been  too  strongly  insisted  upon  in 
the  past,  and  that  while  much  has  been  made  of  their 
differences,  their  points  of  resemblance  have  been  mini- 
mised. The  fact  that  organisms  exist,  which  it  is  difficult  or 
impossible  to  refer  with  certainty  to  either  kingdom,  points 
to  a  fundamental  unity  of  living  substance.  Protoplasm 
in  short  is  the  same  material,  whether  we  call  it  animal  or 
vegetable.  This  being  the  case,  ^its  conditions  of  life  and  its 
immediate  necessities  must  be  practically  the  same,  what- 
ever its  degree  of  differentiation  in  either  direction.  I 
have  tried  to  bring  out  this  identity  of  living  substance 
throughout  the  book,  and  to  indicate  that  apparent  differ- 
ences of  behaviour  and  structural  arrangement  are  to  be 
traced  rather  to  differences  of  environment  and  habit  of 
life  than  to  those  of  constitution.  The  correspondence  of 
the  processes  of  respiration  in  animals  and  plants  has  long 
been  recognised;  many  points  of  similarity  in  those  of 
nutrition  have  been  observed.  The  idea  is,  however,  still 


PKEFACE  TO  THE  FIRST  EDITION  ix 

prevalent  that  plants  live  upon  inorganic  materials  ab- 
sorbed from  the  air  and  from  the  soil.  This  seems  to 
indicate  a  fundamental  difference  between  the  modes  of 
nutrition  of  animal  and  vegetable  protoplasm.  I  have 
endeavoured  to  show  that  this  view  is  erroneous  and  that 
both  are  nourished  similarly. 

I  have  also  tried  to  show  that  the  sensitiveness  of  the 
plant  and  the  animal  is  alike  in  properties,  though  differ- 
ences are  apparent  in  the  direction  of  its  differentiation. 

I  have  avoided  as  far  as  possible  the  discussion  of  con- 
troverted points,  feeling  that  this  would  be  out  of  place  in 
a  work  intended  to  serve  as  an  introduction  to  the  subject. 
Such  matters  are  more  properly  treated  of  in  the  more 
comprehensive  works  to  which  I  have  already  alluded. 

J.  REYNOLDS  GREEN. 
CAMBRIDGE,  June  1900. 


CONTENTS 


CHAPTER  I 

THE   GENERAL   STRUCTURE    OF   PLANTS 

PAGE 

Unicellular  plants  ;  zoogonidia,  yeasts,  bacteria  ;  multicellular  plants  ; 
the  protoplast,  its  structure  and  arrangements  ;  characters  of 
protoplasm  ;  nuclei  and  nucleoli ;  association  of  protoplasts  in 
colonies  ;  slime  fungi ;  coanocytes  ;  arrangements  in  multicellular 
plants — Needs  of  protoplasm ;  its  relation  to  water  ;  formation  of 
vacuoles  ;  relation  of  water  to  the  plant  in  general ;  the  aeration  of 
protoplasm — Connection  of  protoplasts  with  one  another  in  the 
body  of  the  plant 1-16 


CHAPTER  II 

THE   DIFFERENTIATION   OF   THE   PLANT-BODY 

Division  of  labour  the  clue  to  differentiation  of  structure — Formation 
of  protective  tissues  :  epidermis,  cuticle,  periderm,  bark — System 
of  conducting  tissues  ;  vascular  bundles  and  their  distribution — 
Strengthening  tissues  :  collenchyma  and  sclerenchyma ;  the 
different  arrangements  of  them  which  are  met  with — The  stereome 
of  the  plant — The  metabolic  tissues — The  arrangements  for  the 
aeration  of  the  interior  ;  stomata,  lenticels  ....  17-40 

CHAPTER  III 

THE   SKELETON   OF   THE   PLANT 

Necessity  of  a  skeleton  to  support  the  protoplasts  ;  varieties  of  the 
skeleton — Development  of  the  skeleton  as  the  plant  grows — Charac- 
ters of  the  cell-wall ;  cellulose,  its  properties  and  reactions  ;  pectose 
and  related  substances — Arrangement  of  the  solid  matter  and 
the  water  of  the  cell- wall :  hypotheses  of  Naegeli,  Strasburger,  and 
Wiesner — Differentiation  of  the  substance  of  thickened  cell- walls  ; 


xii  VEGETABLE  PHYSIOLOGY 

PAGE 

stratification,  middle  lamella — Lignin  and  its  reactions — Cutin — 
Impregnation  of  cell-walls  with  various  matters — Mucilage — 
Differences  between  temporary  and  permanent  portions  of  the 
skeleton  ....  .  •  41-57 

CHAPTER  IV 

THE   RELATION   OF   WATER  TO   THE   PROTOPLASM   OF 
THE   CELL 

Dependence  of  the  protoplasts  on  water ;  function  of  the  vacuole — 
Renewal  of  the  water  of  the  vacuole — Osmosis — Formation  of  the 
vacuole  as  the  protoplast  develops — Regulation  of  osmosis  by  the 
cell- protoplasm  ;  the  plasmatic  membranes — Movements  of  water 
from  cell  to  cell — Evaporation  into  the  intercellular  spaces — 
Turgescence  and  its  dependence  on  the  protoplasm — Storage  of 
water 58-70 

CHAPTER  V 

THE   TRANSPORT   OF   WATER  IN  THE   PLANT 

Varied  needs  of  different  plants  in  this  respect — Transport  in  a  ter- 
restrial plant — The  ascending  sap — Condition  of  water  in  the  soil ; 
absorption  of  water  a  function  of  the  root-hairs ;  mechanics  of 
the  root-hair — Path  of  the  ascending  stream  ;  forces  causing  the 
movement— Evaporation  of  water  from  the  interior — Influence  of 
the  stream  of  water  upon  the  development  of  the  plant  .  71-82 

CHAPTER  VI 

THE   TRANSPIRATION   CURRENT.      ROOT-PRESSURE. 
TRANSPIRATION 

The  ascending  sap,  sometimes  called  the  transpiration  current — Its 
path ;  methods  of  demonstration — Rate  of  the  transpiration 
current — Causes  of  the  upward  flow ;  root  pressure ;  transpira- 
tion ;  capillarity  ;  pumping  action  of  living  cells  ;  osmotic  action 
of  the  parenchyma  of  the  leaves — Root  pressure ;  its  nature  and 
mode  of  action  ;  bleeding  of  cut  stems  ;  of  entire  plants  ;  measure- 
ment of  root  pressure  ;  conditions  of  the  activity  of  roots  ;  diurnal 
variations  of  root  pressure — Transpiration ;  methods  of  demonstra- 
tion ;  amount  of  water  given  off ;  negative  pressure  in  the  wood 
vessels  ;  character  of  the  evaporation  of  transpiration  ;  regulation 
by  stomata,  their  mode  of  action ;  variations  in  numbers  of 
stomata  ;  conditions  affecting  transpiration  ;  light,  temperature, 


CONTENTS  xiii 

PAGE 

moisture  of  air,  rest — The  Potometer — Suction  of  transpiration — 
Osmotic  action  of  the  parenchyma  of  the  leaves  and  its  effect — 
Regulation  of  all  these  forces  by  the  protoplasm  .  .  83-108 


CHAPTER  VII 

THE   AERATION   OP  PLANTS 

Necessity  of  admitting  oxygen  to  the  protoplasts — The  intercellular 
space  system  ;  its  origin  and  development ;  condition  in  terrestrial 
plants ;  relative  extent  in  roots,  stems,  leaves — Air  reservoirs  in 
aquatic  plants ;  in  Equisetum,  grasses,  rushes,  &c.  ;  mode  of 
formation  of  the  reservoirs — External  orifices  of  the  intercellular 
space  system ;  stomata  and  lenticels — Relative  dimensions  of 
cellular  tissue  and  intercellular  spaces — Movements  of  air  in  inter- 
cellular space  system — Composition  of  the  air  .  .,-...  109-123 


CHAPTER  VIII 

THE  FOOD  OF  PLANTS.   INTRODUCTORY 

True  nature  of  the  food  of  plants — Materials  absorbed  by  plants,  and 
their  relationship  to  actual  food-— Differences  between  food  and 
food  materials — Construction  of  food  from  the  latter — Assimilation 
of  food — Intricacy  of  the  metabolic  processes  of  plants  .  ,  124-131 

CHAPTER  IX 

THE  ABSORPTION   OF   FOOD   MATERIALS   BY   A   GREEN  PLANT 

Examination  of  substances  absorbed  from  the  soil ;  water-culture ; 
destructive  analysis — Classification  of  materials  absorbed — The 
ash  of  plants — Conditions  of  absorption  of  substances  in  the  soil — 
Absorption  of  nitrogen  by  leguminous  plants — Absorption  of 
metallic  compounds  ;  silicon — Absorption  of  carbon  dioxide  from 
the  air ;  its  mechanism 132-145 

CHAPTER  X 

THE  CHLOROPHYLL  APPARATUS 

Formation  of  organic  substances  from  the  inorganic  materials 
absorbed — Chlorophyll — Structure  of  a  chloroplast — Properties  of 
chlorophyll ;  its  absorption  spectrum — Xanthophyll — Erythrophyll 
— Composition  of  chlorophyll — Distribution  of  the  chloroplasts — •. 


xiv  VEGETABLE  PHYSIOLOGY 

PACK 

Relationship  between  the  plastid  and  the  colouring  matter — Leuco- 
plasts — Conditions  of  formation  of  chlorophyll ;  light,  temperature, 
jron — Formation  of  carbohydrates  by  chloroplasts  ;  conditions  of 
their  activity — Theories  of  photosynthesis — Relation  of  starch  to 
the  process — Rays  of  light  made  use  of  in  photosynthesis ;  researches 
of  Engelmann,  of  Tmiriazeff — Inhibition  of  the  chlorophyll  appara- 
tus— Formation  of  organic  substance  in  its  absence  .  .  146-104 


CHAPTER  XI 

THE   CONSTRUCTION  OP  PROTEINS 

Complexity  of  the  composition  of  protein  ;  its  percentage  composition 
— Classification  of  proteins ;  albumins,  globulins,  metaproteins, 
proteoses,  peptones,  proteins  soluble  in  alcohol — Synthesis  of 
proteins  in  plants ;  various  hypotheses — Locality  of  protein  con- 
struction in  the  plant 165-175 

CHAPTER  XII 

THE   CONSTITUENTS   OF  THE  ASH  OF  PLANTS 

Nature  and  composition  of  the  ash — Water-culture  and  the  limitations 
of  its  usefulness  in  the  study  of  the  ash — Classification  of  the 
constituents  of  the  ash — The  selective  power  of  plants — Sulphur 
and  phosphorus — Potassium,  magnesium,  calcium,  iron — Sodium, 
silicon,  chlorine,  bromine,  iodine,  manganese — Accidental  con- 
stituents of  the  ash — Relation  of  nitrogen  and  potassium  to  herbage 
plants 176-188 

CHAPTER  XIII 

OTHER  METHODS  OF  OBTAINING  FOOD 

Partial  or  entire  absence  of  the  constructive  power — Nutrition  of 
saprophytes  —  Insectivorous  plants  —  Utricularia  —  The  pitcher- 
plants — Drosophyllum  —  Pinguicula  —  Dionsea  —  Drosera  — 
Digestion  of  substances  by  Fungi — Commensalism — Symbiosis — 
Mycorhiza — Root  parasites — Parasitism  among  green  plants  and 
Fungi 189-212 

CHAPTER  XIV 

TRANSLOCATION  OF  NUTRITIVE   MATERIALS 

Conditions  of  the  constructive  processes ;  surplus  production  and 
storage — Necessity  of  circulation  of  food  material  in  consequence 
of  localisation  of  construction,  and  intermittence  of  consumption — 


CONTENTS  xv 

PAGE 

Mode  of  transport  of  substances  in  the  plant ;  osmosis  and  diffusion ; 
temporary  storage — Translocatory  and  storage  forms  of  food — The 
so-called  descending  sap — The  pathway  of  translocation  .  213-226 


CHAPTER  XV 

THE   STORAGE   OF   RESERVE   MATERIALS 

Connection  between  transport  and  storage  ;  forms  in  which  food  is 
stored — Reservoirs  of  storage  ;  stems,  roots,  floral  organs — Storage 
of  carbohydrates  ;  starch  grains  and  their  formation  by  chloro- 
plasts  and  leucoplasts,  by  the  cytoplasm ;  glycogen ;  inulin ; 
sugars  ;  cellulose  and  similar  compounds — Storage  of  proteins  ; 
aleurone  grains,  their  composition  and  mode  of  formation  ;  protein 
crystals ;  antecedents  of  gluten — Storage  of  asparagin  ;  glucosides ; 
fats  and  oils — Mode  of  formation  of  the  last-named  group  .  227-247 

CHAPTER  XVI 

DIGESTION   OP  RESERVE   MATERIALS 

Nature  of  digestion — Its  localisation  in  plants — Agents  of  digestion — 
Secretion  of  enzymes — Conditions  of  their  action — Zymogens — 
Differentiation  of  glandular  structures — Classification  of  enzymes 
— Diastase  and  its  action  on  starch — Inulase — Invertase — Glucase 
— Cytase  and  cell- walls — Pectase — Proteoclastic  enzymes — Rennet 
—Enzymes  which  decompose  glucosides — Lipase  and  its  action  on 
fats — Zymase  and  the  production  of  alcohol — Oxidases — Fermenta- 
tive activity  of  protoplasm — Assimilation  ....  248-263 


CHAPTER  XVII 

METABOLISM 

Constructive  and  destructive  processes  ;  anabolism  and  katabolism — 
Constructive  processes  depending  on  katabolism — Secretion — Bye- 
products — Secretion  of  enzymes — Formation  of  cell- wells,  of  starch 
grains,  of  aleurone  grains,  of  fat,  of  chlorophyll,  of  anthocyan — 
Formation  of  resin,  of  alkaloids,  of  acids — Decomposition- products 
of  cellulose  ;  colouring  matters  ;  nectar  ;  etherial  oils  .  .  264-280 

CHAPTER  XVIII 

THE   ENERGY   OF   THE  PLANT 

Preliminary  considerations — The  expenditure  of  energy  in  evaporation, 
in  constructive  processes,  in  movements,  in  radiation,  in  light — 
Source  of  the  energy  of  plants  the  radiant  energy  of  the  sun ;  its 


xvi  VEGETABLE  PHYSIOLOGY 

PAGE 

absorption  by  chlorophyll ;  absorption  of  heat  rays — Fixation  of 
energy — Kinetic  and  potential  energy — Distribution  and  liberation 
of  energy — Dependence  of  the  plant  upon  oxygen  ;  absorption  of 
oxygen  and  exhalation  of  carbon  dioxide ;  apparatus  to  demon- 
strate these  processes — Loss  of  weight  during  respiration — Varia- 
tions in  the  respiratory  activity — Relation  between  the  absorption 
of  oxygen  and  the  exhalation  of  carbon  dioxide ;  the  respiratory 
quotient — Exhalation  of  water  during  respiration — Respiration  a 
function  of  protoplasm — Conditions  affecting  respiration ;  tempera- 
ture, light,  differences  of  gaseous  pressure  and  of  nutritive 
materials — Relation  of  respiratory  processes  to  local  utilisation  of 
potential  energy — Oxidative  actions  other  than  respiration — Intra- 
molecular or  anaerobic  respiration — Fermentation — Anaerobic 
plants 281-309 


CHAPTER  XIX 

GROWTH 

Relation  of  growth  to  constructive  metabolism — Definition  of  growth 
— Distribution  of  growth — Conditions  necessary  for  growth  ;  plastic 
materials,  turgescence,  temperature,  oxygen — The  grand  period  of 
growth — Growth  of  a  cell  and  of  a  multicellular  organ — The  region 
of  growth  in  the  latter — Daily  period  of  growth  in  length — The 
Auxanometer — Variations  in  growth  ;  hyponasty  and  epinasty  ; 
nutation  and  circumnutation — Tensions  accompanying  growth — 
Rectipetality  .  310-324 

CHAPTER  XX 

TEMPERATUEE   AND  ITS   CONDITIONS 

Range  of  temperature  through  which  the  vital  processes  proceed; 
photosynthesis,  germination — Causes  and  effects  of  fluctuations  of 
temperature — Influence  of  the  light  rays  on  temperature ;  impor- 
tance of  anthocyan — Absorption  of  heat  by  conduction — Dissipa- 
tion of  heat  in  evaporation  of  water — Radiation — Nyctitropic 
movements — Loss  and  gain  of  heat  by  conduction — Regulation  of 
heat— Power  of  resistance  to  extremes  of  temperature  .  325-335 


CHAPTER  XXI 

INFLUENCE   OP  THE   ENVIRONMENT   ON  PLANTS 

Characters  of  aquatic  plants  ;  influence  of  a  watery  environment  on 
structure — Xerophytes  and  their  peculiarities — Alpine  plants — 
Epiphytes — Parasites — Insectivorous  plants  .  .  .  336-351 


CONTENTS  xvii 

CHAPTEK  XXII 

THE  PROPERTIES  OF  VEGETABLE  PROTOPLASM 

PAGE 

Adaptability  of  plants  to  their  surroundings — Contractility — Ciliary 
and  amoeboid  movement — Locomotion — Movements  of  rotation 
and  circulation — Turgor  and  its  maintenance — Mobile  condition  of 
protoplasm — Rhythm  and  its  manifestations — Irritability  and  its 
conditions — Tone — Phototonus — Thermotonus — Tonic  influence 
of  light — Etiolation — Influence  of  too  brilliant  illumination; 
paraheliotropism,  apostrophe  and  epistrophe — Photo-epinasty — 
Regulating  action  of  light  on  growth — Conditions  of  health — 
Acclimatisation 352-374 

CHAPTER  XXIII 

STIMULATION   AND   ITS   RESULTS 

Response  of  an  organism  to  changes  in  its  surroundings — Nature  of 
stimulation — Purposeful  character  of  the  response — Stimulation  of 
light — Nyctitropic  movements,  their  conditions  and  purpose — 
Mechanism  of  the  movements — Effect  of  incidence  of  lateral  light 
— Heliotropism — Stimulus  of  gravitation ;  geotropism — The  Klino- 
stat — Knight's  wheel — Stimulus  of  contact — Behaviour  of  various 
organs  in  relation  to  this  form  of  stimulation — The  root — Twining 
stems  and  tendrils — Hydrotropism — Chemical  stimuli — Chemotaxis 
— Induced  rhythm 375-405 

CHAPTER  XXIV 

THE  NERVOUS  MECHANISM  OF  PLANTS 

The  purposeful  responses  of  plants  to  stimulation  ;  relation  of  stimulus 
to  effect — Nature  of  nervous  mechanisms — Sense  organs  and  their 
differentiation — Motor  mechanisms  of  plants — Contraction — 
Regulation  of  supply  of  water  to  the  cell — Glandular  organs — Con- 
duction of  impulses  ;  continuity  of  protoplasm — Co-ordination  of 
impulses — Latent  period  of  stimulation — After-effects — Fatigue — 
Anaesthetics — Comparison  of  nervous  mechanisms  of  plants  and 
animals  406-419 


CHAPTER  XXV 

REPRODUCTION 

Distinction  between  the  individual  protoplast  and  the  colony  or  plant 
— Process  of  multiplication  of  protoplasts ;  gemmation,  karyo- 
kinesis,  formation  of  cell- walla  ;  free-cell  formation — Vegetative 


xviii  VEGETABLE  PHYSIOLOGY 

PAGE 

propagation — Formation  of  asexual  reproductive  cells,  spores  or 
gonidia  ;  zoocoenocytes — Development  of  sexual  cells  or  gametes  : 
planogametes  and  conjugation  ;  male  and  female  cells  ;  anthero- 
zoids  and  oospheres — Gametangia  and  their  varieties — Fertilisation 
— Alternation  of  generations — Gametophytes  and  sporophytes — 
Heterospory  and  its  consequences — The  seed  and  its  formation  420-447 

CHAPTER  XXVI 

REPRODUCTION  (continued) 

Pollination  and  its  mechanisms — Advantages  of  cross-pollination — 
Dichogamy,  protandry,  protogyny — Diclinism — Heterostylism  or 
dimorphism — Prepotency — Self -sterility — Self-pollination ;  cleisto- 
gamy — Mechanism  of  fertilisation ;  the  growth  of  the  pollen-tube 
— Hybridisation — Results  of  fertilisation  ;  formation  and  ripening 
of  fruits  and  seeds — Germination  of  the  seed — Apospory ;  apogamy ; 
parthenogenesis 448-460 

INDEX        .  461-470 


LIST  OF  ILLUSTRATIONS 


no. 

1.  Zoospore  of  Ulolhrix 

2.  Yeast  Plants 2 

3.  Bacteria 3 

4.  Plasmodium  of  a  Myxomyctte,    . 

5.  Vegetable  Cells  (Young)                 5 

6.  Vegetable  Cells  (Adult)             ...                   ...  5 

7.  Cells  exhibiting  Rotation,  from  Elodca 7 

8.  Cells  of  Tradescantia,  showing  circulation    ...  .7 

9.  Structure  of  the  Nucleus 7 

10.  Colonies  of  Protococcus • 

11.  Volvox  Globator 10 

12.  Ccenocytic  Suspensor  of  Orobus        ..... 

13.  Filaments  of  Nostoc 12 

14.  Pediastrum • 

15.  Vegetable  Cells  (Young)       .                   12 

16.  Vegetable  Cells  (Adult)  .... 

17.  Continuity  of  Protoplasm  in  Seed        .                    .                    .      .  16 

18.  Continuity  of  Protoplasm  in  Seaweed        .          .  .  .16 

19.  Thallus  of  Pdvetia 22 

20.  Stem  of  Sphagnum           ...                            ...  23 

21.  Stem  of  Common  Moss         .                   23 

22.  Section  of  Blade  of  Leaf           .         .                   .                   .          .  23 

23.  Cork  Cells ...  24 

24.  Bark  of  Oak            ..-.-. 24 

25.  Collenchyma 25 

26.  Exodermis  of  Root .25 

27.  Diagram  of  Course  of  Vascular  Bundles  in  a  Dicotyledonous  Plant .  27 

28.  Venation  of  Leaf .27 

29.  Section  of  Rhizome  of  Fern 30 

30.  Section  of  Leaf  of  Pinus .31 

31.  Vascular  Bundle  of  Monocotyledon        .          .          .          .          .      .  32 

32.  Different  Arrangements  of  Stereome  in  Herbaceous  Plants 

33.  Chloroplasts  in  Cell 

34.  Section  of  Stem  of  Potamogeton 

35.  Cortex  of  Root .      .  37 

36.  Section  of  Blade  of  Leaf    .                                                                      -  37 

37.  Stomata  on  Lower  Surface  of  Leaf         .                                       .      .  38 

38.  Section  of  Epidermis  of  Leaf 

39.  Section  of  a  Lenticel             .          .          .          .          .          •          .      .  39 


xx  VEGETABLE  PHYSIOLOGY 

FIO.  PAGE 

40.  Section  of  Dicotyledonous  Stems  of  three  ages .  .  .43 

41.  Embryo  of  Orobus       .  .     .     44 

42.  Stratification  in  Cell- walls         .  .  .     49 

43.  Longitudinal  Section  of  Vascular  Bundle  of  Sunflower  Stem          .     50 

44.  Wood-cells,  showing  Middle  Lamella          .          .          .          .  .51 

45.  Section  of  Epidermis  of  Leaf         ...  ...     54 

46.  Cork  in  Twig  of  Lime       .         .  ,55 

47.  Section  of  a  Lenticel  .  .      .     55 

48.  Crystals  in  Wall  of  Cell  of  Bast         .  .  .     56 

49.  Cystolith  of  Ficus        ...  .      .     56 

50.  Apparatus  to  show  the  process  of  Osmosis          ...           .60 
61.  Young  Vegetable  Cells 62 

52.  Adult  Vegetable  Cells       ...  .63 

53.  Cells  undergoing  Plasmolysis         .  64 

54.  Rootlets  with  Root-hairs  .  .  .     73 

55.  Root-hair  in  contact  with  particles  of  Soil  .          .          .      .     74 

56.  Section  of  Young  Root  .75 

57.  Diagram  of  Course  of  Vascular  Bundles  in  a  Dicotyledonous  Plant     76 

58.  Veins  of  a  Leaf        .  .     77 

59.  Ending  of  a  Vascular  Bundle  in  a  Leaf          ...  .78 

60.  Intercellular  Spaces  in  Leaf      ......  .78 

61.  Stomata  on  Lower  Surface  of  Leaf  .          .          .  80 

62.  Apparatus  for  the  Estimation  of  Root-pressure  .          .  .89 

63.  Apparatus  to  demonstrate  Transpiration        .          .          .  94 

64.  Section  of  Blade  of  Leaf 95 

65.  Apparatus  to  show  Dependence  of  Withering  on  Loss  of  Water     .     96 

66.  Stomata  on  Lower  Surface  of  Leaf   .....          .98 

67.  Section  of  a  Stoma  98 

68.  Darwin's  Potometer 103 

69.  Apparatus  to  show  the  Suction  of  Transpiration  .         .     .   105 

70.  Ending  of  a  Vascular  Bundle  in  a  Leaf       .....   106 

71.  Formation  of  Intercellular  Spaces Ill 

72.  Intercellular  Spaces  in  Root  .         .*        .         .         .          .111 

73.  Intercellular  Spaces  in  Leaf  . 112 

74.  Section  of  Leaf  of  Isoetes          ...  .         .  .112 

75.  Section  of  Rhizome  of  Marsilea   .          . .        .         .          .  .    113 

76.  Section  of  Stem  of  Potamogeton .114 

77.  Section  of  Stem  of  Equisetum 115 

78.  Section  of  Stem  of  Juncus .116 

79.  Section  of  a  Lenticel  117 

80.  Apparatus  to  show  Continuity  of  Intercellular  Spaces  in  a  Leaf  .    118 

81.  Section  of  Leaf  of  Heath  119 

82.  Root  of  a  Leguminous  Plant  with  its  Tubercles  .          .  .    140 

83.  Section  of  Blade  of  Leaf 144 

84.  Absorption  Spectra  of  Chlorophyll  and  Xanthophyll  .  .    148 

85.  Chloroplasts  in  Cell  150 

86.  Section  of  Leaf  of  Beta  151 

87.  Section  of  Stem  of  Equisetum 152 


LIST  OF  ILLUSTRATIONS  xxi 


Pia. 

88.  Apparatus  to  show  the  Evolution  of  Oxygen  by  a  Green  Plant  .   156 

89.  Plants  of  Buckwheat  cultivated  in  various  Nutritive  Solutions    .  179 

90.  Aleurone  Grains  in  Cell  of  Ricinus       .          .....  182 

91.  Bladderworts  (Utricularia)     .                    .....  193 

92.  Traps  of  Utricularia            .          .                    .                    ...  194 

93.  Pitcher  of  Sarrac-enia      .......          .195 

94.  Pitcher  of  Nepenthes            ........  196 

95.  Leaf  of  Drosera               ........  198 

96.  Leaf  of  Dioncea          .........  199 

97.  Section  of  Lichen            ........  202 

98.  Mycorhiza  on  Beech  Root            .......  204 

99.  Plant  of  Thesium            ........  206 

100.  Sucker  of  Thesium  ........   207 

101.  Plant  infested  with  Dodder    ......          .   209 

102.  Haustoria  of  Dodder  ........   210 

103.  Haustoria  of  Phytophthora      .         .         .  .         .          .211 

104.  Starch  Grains  in  Chloroplast       .....         .      .   220 

105.  Section  of  Stem  of  Ricinus  .....  .231 

106.  Section  of  Stem  of  Tilia  three  years  old       .....   232 

107.  Starch  Grains  in  Chloroplast  .....          .   234 

108.  Starch  Grains  in  Cell  of  Potato  .....      .235 

109.  Starch  Grain  of  Potato  .......   235 

110.  Compound  and  Semi-compound  Starch  Grains  .          .      .   236 

111.  Laticiferous  Cell  of  Euphorbia         .         .         .         .         .          .237 

112.  Leucoplasts  of  Phajus  .         .         .         .         .         .      .   237 

113.  Inulin  Sphsero-crystals  .......   239 

114.  Aleurone  Layer  of  Barley  .......   240 

115.  Cells  of  Embryo  of  Pea  .......   241 

116.  Formation  of  Aleurone  Grains     .......   241 

117.  Aleurone  Grains  of  Lupin        ......  .   242 

118.  Aleurone  Grains  of  Ricinus          .......   242 

119.  Section  of  Oat  Grain  .......   252 

120.  Epithelium  of  Scutellum  .......   252 

121.  Aleurone  Layer  of  Barley       ......          .   253 

122.  Gland  of  Drosera  ........   253 

123.  Corrosion  of  Starch  Grains  by  Diastase  ....   255 

124.  Glandular  Hairs  of  Primula         .......   275 

125.  Glandular  Hairs  of  Hop          .....         ..275 

126.  Oil  Reservoirs  of  Hypericum       .......   278 

127.  Bast-Cell  of  Ephedra  .         .         .         .         .         .          .278 

128.  Crystals  of  Oxalate  of  Calcium  in  Cells         .          .          .          .      .   279 

129.  Cystolith  of  Ficus          .         .         .         .         .         .         .          .279 

130.  Absorption  Spectra  of  Chlorophyll  and  Xanthophyll  .       .  286 

131.  Apparatus  to  show  the  Absorption  of  Oxygen  by  a  Green  Plant    .   293 

132.  Apparatus  to  show  Exhalation  of  Carbon  Dioxide  by  Germinating 

Seeds  ..........   294 

133.  Longitudinal  Section  of  Growing  Point  of  Root         .          .  .311 

134.  Section  of  Blade  of  Leaf  .  312 


xxii  VEGETABLE  PHYSIOLOGY 

F10.  PAGE 

135.  Section  of  Stem  of  Rush 313 

136.  Adult  Vegetable  Cells  ....  .     .  315 

137.  Region  of  Growth  in  Root  of  Bean          .         .         .         .          .318 

138.  Pfeffer's  Auxanometer 319 

139.  Air  Passages  in  Potamogeton .337 

140.  Leaf  of  Isoetes ...   338 

141.  Petiole  of  Water-lily  ...  ...   339 

142.  Rhizome  of  Marsilea 341 

143.  Water-gland  of  Saxifraga 343 

144.  Leaf  of  Heath  .  .  .  .     .   345 

145.  Suckers  of  Thesium       .          .  .  .  .348 

146.  Section  of  a  Sucker  349 

147.  Zoospore  of  Ulothrix       .         .  .353 

148.  Plasmodium  of  a  Myxomycete 354 

149.  Cells  from  Leaf  of  Elodea 357 

150.  Cells  from  Hair  of  Tradescantia 357 

151.  Leaf  of  Telegraph  Plant 362 

152.  Pulvinus  of  Mimosa 363 

153.  Desmodium  gyrans,  day  and  night  position       ....  380 

154.  Nicotiana  glauca,  day  and  night  position  .          .  .  381 

155.  Pulvinus  of  Mimosa       .....  .  .383 

156.  Darwin's  Klinostafc .   389 

157.  Section  of  Sucker  of  Thesium 397 

158.  Haustoria  of  Cuscuta 398 

159.  Leaf  of  Dioncea *399 

160.  The  same 410 

161.  Continuity  of  Protoplasm  through  the  Cell-wall        .          .  .413 

162.  Yeast  Plants  422 

163.  Stages  in  Karyokinetic  Division  of  the  Nucleus         .          .  .  423 

164.  Zoospore  of  Ulothrix 427 

165.  Gonidangia  of  Achlya 427 

166.  Coenocyte  of  Mucor  .         .         .         .         .          .          .      .   428 

167.  Part  of  Hy menial  Layer  of  Peziza  .....   428 

168.  Stylogonidia  of  Eurotium  429 

169.  Filament  of  Ulothrix  with  Gametes  escaping     ...          .430 

170.  Oogonium  of  Fucus  432 

171.  Oosphere  and  Antherozoids  of  Fucus       .          .          .          .  .432 

172.  Procarpium  of  a  Red  Seaweed 433 

173.  Archegonium  of  Fern  ......  .   434 

174.  Prothallium  of  Fern 437 

175.  Germination  of  Microspores  of  Salvinia  ....   438 

176.  Germination  of  Megaspore  of  Salvinia 439 

177.  Germination  of  Megaspore  of  Selagindla  ....   440 

178.  Ovule  of  Pinus 441 

179.  Ovule  of  an  Angiosperm .441 

180.  Antherozoids  of  Moss  and  Fern 445 

181.  Antheridium  of  Fern 445 

182.  Archegonium  of  Fern 446 


VEGETABLE     PHYSIOLOGY 

CHAPTEK  I 

THE    GENERAL    STRUCTURE    OF   PLANTS 

Examination  of  the  body  of  every  living  organism  shows  us 
that  it  is  composed  of  different  materials,  which  exhibit  a 
great  deal  of  variety  in  the  ways  in  which  they  are  arranged. 
These  different  materials  fall  very  naturally  into  two  classes, 
which  include  respectively  the  living  substance  itself,  and 
various  constituents  of  the  body  which  have  been  con- 
structed by  it.  The  relative  proportions  in  which  these  two 
classes  of  materials  exist  vary  very  greatly  in  different 
organisms  ;  in  some  of  the  simplest  forms  indeed  we  can 
discern  nothing  structural  except  the  living  substance  itself. 
In  others  the  materials  constructed  by  the  latter  are  much 
the  most  conspicuous. 

When  we  study  the  life  history  of  the  simplest  or  the 
most  complex  plant  with  which  we  can  become  acquainted, 
we  find  that  at  some  time  or  other  in 
its  existence  it  is  found  in  the  form  of 
a  minute  portion  of  jelly-like  material 
which    is    endowed    with    life.     Some- 
times this  piece  of  living  substance  is 
motile,  and  can  swim  freely  about  in 
water  by  means  of  certain  thread-like 
appendages  which  it  possesses  (fig.  1). 
Such    structures    occur    almost    exclu- 
sively among  the  lowest  forms  of  plants,  particularly  the 
seaweeds.     They   are   known   as   zoospores,   or   zoogonidia, 

I 


2  VEGETABLE  PHYSIOLOGY 

and  are  produced  in  large  numbers.  In  other  cases  the 
little  mass  of  living  substance  is  not  capable  of  locomotion, 
but  may  be  found  floating  about  in  water,  or  enclosed  in 
particular  cavities  in  its  parent  plant. 

The  jelly-like  substance  of  which  these  bodies  are  com- 
posed is  living  and  capable  of  carrying  out  all  the  functions 
necessary  for -its  life,  growth,  and  multiplication.  It  is 
called  protoplasm,  and  each  portion  of  protoplasm  which  is 
thus  capable  of  independent  existence  is  known  as  a  vege- 
table cell,  or  protoplast. 

These  free-swimming  organisms  are  not  protected  by 
any  coating,  but  every  part  of  their  surface  is  in  complete 
contact  with  the  water  in  which  they  live.  This  condition 


FIG.  2.— SACOHAROMYCES  CEREVISIJE,  OB  YEAST-PLANT,  AS  DEVELOPED 
DURING  THE  PROCESS  OF  FERMENTATION.     X  300. 
a,  b,  c,  d,  successive  stages  of  cell-multiplication. 

is,  however,  exceptional.  Usually  the  protoplast  is  encased 
in  a  colourless  homogeneous  membrane  of  extreme  tenuity 
which  is  known  as  its  cell-wall.  Examples  of  unicellular 
organisms  of  this  kind  are  found  in  great  numbers  among 
the  fungi,  the  Yeasts  (fig.  2)  and  the  Bacteria  (fig.  3)  being 
exceptionally  numerous.  Such  plants  may  be  motile  or  non- 
motile,  a  few  of  the  bacteria  being  furnished  with  thread- 
like appendages,  known  as  cilia  or  ftagella,  which  are 
similar  in  most  respects  to  those  of  the  zoospoores  already 
mentioned.  These  plants  show  a  little  more  differentiation 
than  the  others,  the  cell-wall  constituting  a  kind  of  exo- 
skeleton  for  the  protoplasm,  and  being  at  once  supporting 
and  protective. 

More  complex  organisms  consist  of  two  or  more  proto- 
plasts united  together  in  various  ways.    The  number  of 


THE  GENEEAL 

these  masses  of  p 
two,  or  may  be  en 
the  gigantic  seawe 
trees  which  abound 
Whether  the  pi 
same  fundamental  arr 
certain  number  of  p: 
each  other,  supported 
shows    a   wonderful   variety 
depending  on  the  manner  of  life  o 
which  it  forms  so  large  a  part.     In 


PLANTS        3 


small  as 


Fia.  3. — FIGURES  OF  DIFFERENT  BACTERIA. 
(After  Cohn  and  Sachs.    Very  highly  magnified.) 

1,  Sarcina ;  2,  Bacillus ;  3,  Spirillum  ;  4,  Spirillum  with  flagella  ; 
5,  6,  7,  Micrococcus.     (Single,  in  strings,  and  in  groups.) 

protoplast  is  usually  found  occupying  a  particular  cavity 
which  is  formed  by  its  cell-walls,  and  communicating  with 
its  neighbours  on  all  sides  by  delicate  prolongations  of 
living  substance  which  extend  through  the  walls  of  con- 
tiguous chambers.  Each  chamber  is  often  called  a  cell,  but 
it  is  preferable  to  restrict  this  term  to  the  protoplast  which 
occupies  it. 

In  dealing  with  the  physiology  of  the  plant,  it  is  the 
living  substance  which  should  first  engage  our  attention, 
though  the  arrangements  of  the  supporting  structures  or 
skeleton  exhibit  the  greatest  variety.  We  have  seen  that 
in  the  simplest  forms  of  plants  the  living  substance  may 

1* 


4  VEGETABLE  PHYSIOLOGY 

exist  without  any  cell-membrane,  and  may  be  freely  motile, 
swimming  in  water  by  means  of  cilia.  The  absence  of  the 
cell-membrane  can  also  be  observed  in  certain  peculiar 
fungi,  which  are  to  be  found  creeping  over  moist  surfaces 
without  such  appendages  (fig.  4).  These  are  known  as 
the  slime-fungi  or  Myxomycetes.  In  many  respects  they 
approach  very  near  to  one  of  the  humblest  animals,  the 
Amoeba.  They  have  hardly  any  structure,  appearing  like 


Fiu.  4.— PORTION  OF  A  PLASMODIUM  OF  A  Myxomycete.     x  300. 
(After  De  Bary.) 

a  lump  of  transparent  jelly,  the  whole  mass  being  called  a 
plasmodium.  They  have  the  power  of  extruding  a  certain 
portion  of  their  substance  in  the  form  of  a  blunt  process 
known  as  a  pseudopodium,  and  by  means  of  these  pseudo- 
podia  they  can  creep  slowly  over  the  surface  on  which  they 
are  lying.  The  naked  condition  is,  however,  exceptional 
in  plants.  In  most  of  those  which  are  unicellular  the 
living  substance  is  covered  by  its  delicate  membrane,  and 
it  may  either  occupy  all  the  space  inside  the  latter, 
or  may  have  in  its  interior  a  cavity  or  vacuole,  which  is 


THE  GENERAL  STRUCTURE  OF  PLANTS        5 

tilled  with  a  watery  fluid.     In  the  multicellular  plants  each 
chamber  during  life  contains  its  own  protoplast  or  little  mass 


FIG.  5.  —  VEGETABLE  CELLS. 

A,  very  young  ;    B,  a  little  older,  showing  commencing  formation  of  vacuole. 
p,  protoplasm  ;   n,  nucleus  ;   v,  a  vacuole. 

of  protoplasm,  which  is  connected,  as  already  mentioned, 

with  its  neighbours  on  all  sides      In  such  cavities  the  proto- 

plast when  young  usually  occupies  the  whole  of  the  interior 

(fig.  5,  A),  but  when  they  are  adult  it  generally  lies  as  a 

peripheral  layer  round  the  wall,  to 

which  it  is  closely  pressed,  while  a 

central  vacuole  occupies  the  greater 

space  of  the  cavity  enclosed  by  the 

cell  -walls  (fig.  6).     Sometimes   the 

vacuole   is  crossed   by  a  number  of 

bridles    or    strands    of    protoplasm, 

which  generally  pass  from  a  some- 

what central  spot  to  the  periphery. 

The  protoplasm  is  transparent,  but 

somewhat    granular    in   appearance, 

and  is  saturated  with  water.     Some- 

where   in    its    Substance,  whether    it     FlG-    6.—  ADULT   VEGETABLE 
<in        ,1  11  .,  .,  CELLS.       x    500.      (After 

nils    the    cell  -cavity    or   not,  there        Sachs.) 
exists  a  special    differentiated    por- 

,  •  11     i    .-,  7  .. 

tion  called  the  nucleus,     bometirnes, 

but  only  in  particular  cells,  the  pro- 

toplasm   contains    other    differentiated    portions,    distinct 

from   the    rest    of    the    substance,   which   are   known    as 


/»,  ceii-waii;  p,  protoplasm; 

k,    k',   nucleus,   with   nu- 

cieoii  ;  «  «', 


6  VEGETABLE  PHYSIOLOGY 

plastids.  The  bulk  of  the  living  substance,  to  distinguish 
it  from  these  specialised  portions,  is  usually  called  the 
cytoplasm.  It  is  not  of  the  same  consistency  throughout, 
its  outer  portion,  which  is  in  contact  with  the  cell-wall 
and  is  somewhat  denser  in  character,  being  known  as  the 
ectoplasm.  A  similar  firm  layer  may  frequently  be  detected 
round  the  vacuole.  These  are  not,  however,  to  be  confused 
with  the  cell-membrane  or  cell- wall,  being  particular  layers 
of  the  cytoplasm. 

The  exact  chemical  composition  of  protoplasm  cannot 
be  ascertained,  as  analysis  involves  its  death,  and  this  is 
attended  by  changes  in  its  substance.  It  contains  carbon, 
hydrogen,  oxygen,  nitrogen,  and  probably  sulphur  and 
phosphorus,  but  we  are  quite  unable  to  say  in  what  different 
combinations  they  exist  within  it.  Enclosed  in  it  are 
always  varying  quantities  of  organic  substances  such  as 
proteins,  carbohydrates,  and  fats,  and  small  quantities  of 
various  inorganic  and  organic  salts.  The  substance  of  the 
protoplasm  has  been  thought  either  to  be  arranged  in 
the  form  of  a  network,  these  various  bodies  occupying  the 
meshes,  or  to  have  a  foamy  structure  much  like  that  pro- 
duced by  vigorously  stirring  a  mixture  of  oil  and  water.  The 
various  substances  alluded  to  as  occurring  in  close  relation- 
ship to  it  are  connected  with  the  nutritive  and  other  vital 
processes  of  the  cell,  or  its  metabolism,  and  hence  differ 
greatly  in  nature  and  amount  from  time  to  time. 

In  the  case  of  the  free-swimming  protoplasts,  with 
which  we  began  the  study  of  protoplasm,  we  saw  they  were 
in  active  motion.  As  the  protoplasts  become  enclosed  in 
cell-walls  this  motility  is,  of  course,  less  and  less  obvious  ; 
indeed  in  most  cells  it  cannot  be  distinguished  at  all.  There 
is  reason  to  suppose,  however,  that  protoplasm,  wherever 
existing,  is  in  active,  though  imperceptible,  motion.  In 
many  of  the  constituent  cells  of  some  of  even  the  higher 
plants  this  motility  can  be  observed,  particularly  where  the 
protoplasm  has  a  granular  appearance.  In  certain  of  the 
cells  forming  the  leaves  of  many  aquatic  plants,  e.g.  Vallis- 


THE  GENEEAL  STRUCTURE  OF  PLANTS 


neria,  Nitella,  Elodea  (fig.  7),  and  others,  a  streaming  move- 
ment of  the  granules  the  protoplasm  contains  can  be  detected 


FIG.   7. — CELLS   FROM  THE   LEAF  OF 
Elodea.     X  500. 

n,  nucleus;  p,  protoplasm,  in  which 
are  embedded  numerous  chloro- 
plasts.  The  arrows  show  the 
direction  of  the  movement  of  the 
protoplasm. 


FIG.  8. — Two  CELLS  FROM  A 
STAMINAL  HAIR  OF  Trades- 
cantia.  X  300. 

The  arrows  show  the  direction 
of  the  movement  of  the  proto- 
plasm. 


under  a  high  power  of  the  microscope.  In  other  plants  of 
terrestrial  habit,  e.g.  certain  cells  of  Tradescantia  and 
Chelidonium,  a  similar  streaming  of 
the  protoplasm  is  observable  (fig.  8). 
Such  movements  are  spoken  of  as 
rotation  when  the  current  flows  uni- 
formly round  the  cell,  or  as  circu- 
lation when  the  path  has  a  more 
complicated  course. 

It  has  been  mentioned  that,  with 
very  rare  exceptions,  all  cells  con- 
tain  a  specially  differentiated  portion 
of  protoplasm,  known  as  the  nucleus 
(figs.  6  and  9).  This  structure  does 
not  occupy  a  very  definite  position  in  the  cell,  but  not 
infrequently  is  found  almost  in  the  centre.  If  the  whole 


THE  CHROMATIN  THREADS. 

X  1000. 

a,  threads ;  b,  nucleolus. 


8  VEGETABLE  PHYSIOLOGY 

of  the  space  is  not  filled  with  protoplasm,  the  part  in 
which  the  nucleus  lies  is  connected  with  the  lining 
layer  by  means  of  strands  or  bridles.  In  other  cases 
the  nucleus  is  embedded  in  some  part  of  the  lining  layer 
itself.  This  body  has  a  more  definite  structure  than  the 
rest  of  the  cytoplasm  ;  it  is  bounded  at  the  surface  by  a 
delicate  membrane,  which  is  thought,  however,  to  be  a 
denser  layer  of  the  protoplasm  of  the  cell,  rather  than  to 
belong  to  the  nucleus  itself.  Within  this  nuclear  membrane 
are  found  two  substances  which  differ  from  each  other  in 
their  power  of  staining  with  various  reagents.  The  bulk 
of  the  nucleus  is  composed  of  a  semi-fluid  material  known 
as  nucleoplasm,  in  which  is  embedded  a  network  of  fibrils 
or  a  long  much-coiled  thread.  The  fibrils,  or  the  thread, 
are  composed  of  a  hyaline  substance  in  which  lie,  close  to 
each  other,  a  number  of  granules  which  stain  deeply  with 
many  colouring  matters.  The  threads  contain  these 
granules  in  such  large  proportion  that,  except  with  very 
high  magnification,  the  latter  cannot  be  distinguished,  and 
consequently  the  whole  fibril  appears  stained.  The  fibrils 
are  generally  said  to  be  composed  of  chromatin,  the  name 
having  reference  to  nothing  more  than  this  reaction  to 
stains. 

One  or  more  small  deeply  staining  bodies,  termed 
nucleoli,  are  found  in  each  nucleus,  sometimes  being  very 
prominent,  and  at  other  times  hardly  distinguishable  from 
the  nodes  of  the  fibrillar  network  or  the  crossings  of  the 
coiled-up  thread  (figs.  6,  ft  ft,  and  9,  b).  Chemically  the 
nucleus  resembles  the  rest  of  the  protoplasm  to  a  consider- 
able extent.  It  contains,  however,  a  material  known  as 
nuclein,  of  which  phosphorus  is  a  constituent.  It  is  not 
known  how  the  nuclein  is  related  to  the  rest  of  the  nuclear 
substance,  but  it  appears  to  be  present  in  the  thread  or 
fibrillar  network  and  not  in  the  general  nucleoplasm. 

It  is  of  such  protoplasts  or  aggregations  of  small  portions 
of  living  substance  that  all  plants  are  built  up.  There  is, 
however,  a  wonderful  variety  in  the  relative  arrangements 


THE  GENEEAL  BTEUCTUBB  OF  PLANTS        9 


FIG.  10.— COLONIES  OF  Protococcus. 
X  750. 


of  these  units  of  construction,  a  variety  which  finds  its 
expression  in  the  multiplicity  of  existing  forms,  and  the 
differences  of  dimensions  which  various  organisms  exhibit. 

The  simplest  plants,  as  we  have  seen,  are  unicellular, 
and  many  remain  in  this  condition  throughout  the  whole  of 
their  existence.  When  they  have  attained  a  certain  size 
the  cell  or  protoplast  divides  into  two.  Sometimes  these 
two  become  separated  from  each  other,  and  we  have  two 
plants  where  but  one  existed  before.  Plants  with  this  habit 
remain  unicellular,  and  the 
division  of  the  cell  is  equiva- 
lent to  the  reproduction  of  the 
plant.  The  unicellular  condi- 
tion in  other  cases  is  transitory, 
and  the  plant  soon  comes  to 
consist  of  two,  four,  or  more 
cells,  in  consequence  of  the 
products  of  each  division  re- 
maining attached  together. 

We  get  in  this  way  a  small  colony  of  cells,  each  like  the  others 
both  in  structure  and  in  function.  When  the  power  of 
division  is  limited  the  resulting  colony  consists  of  a  limited 
number  of  cells,  and  is  often  found  surrounded  by  a  common 
cell- wall  or  membrane.  This  condition  is  seen  in  such 
plants  as  Chroococcus,  Protococcus,  and  other  humble  Algae 
(fig.  10).  A  colony  of  somewhat  higher  type,  though  still 
of  microscopic  size,  is  found  in  the  form  of  a  hollow  sphere 
(fig.  11),  the  wall  of  which  is  one  cell  thick  (fig.  11,  A).  This 
organism,  known  as  Volvox,  shows  a  little  higher  differentia- 
tion than  those  last  described,  the  cells  being  furnished 
with  cilia  by  means  of  which  the  little  sphere  can  propel 
itself  through  the  water. 

In  other  cases  the  association  of  a  number  of  protoplasts 
is  not  complicated  by  the  formation  of  any  cell-wall.  Fig. 
4,  A  shows  an  aggregation  of  a  number  of  naked  proto- 
plasts which  have  combined  to  form  a  plasmodium.  These 
organisms  are  found  creeping  about  upon  moist  surfaces ; 


10  VEGETABLE  PHYSIOLOGY 

they  form  the  group  known  as  the  Myxomycetes  or  slime- 
fungi.  One  species,  Mlwlium,  is  found  frequently  among 
the  refuse  of  tanyards  and  is  known  as  '  flowers  of  tan.' 
These  fungi  pass  the  greater  part  of  their  life  without 


FIG.  11. — VOLVOX  GLOBATOB.    (After  Kny.)     x   120. 
A,  section  of  a  portion  of  the  wall  of  tho  sphere.     X   1000. 

possessing  any  cell-walls,  only  forming  them  indeed  in  con- 
nection with  their  processes  of  reproduction. 

A  third  mode  of  arrangement  of  a  colony  of  proto- 
plasts is  found  in  the  so-called  Coenocytes  (fig.  12).  In  this 
type  of  structure  which  is  represented  by  several  very 


THE  GENERAL  STRUCTURE  OF  PLANTS      11 


important  seaweeds  and  by  a  large  number  of  fungi,  as  well 
as  by  particular  parts  of  some  of  the  flowering  plants,  we 
have  a  number  of  protoplasts  arranged  together  over  the 
inner  surface  of  a  common  cell-wall.  The  separate  proto- 
plasts are  often  in  such  close  contact 
with  each  other  that  their  several 
outlines  cannot  be  detected.  They 
have  the  appearance  of  a  mass  of 
protoplasm  lining  the  wall  of  a 
hollow,  generally  tubular,  cavity, 
and  having  a  large  number  of  nuclei 
embedded  in  the  mass.  The  pre- 
sence of  a  number  of  nuclei  indicates 
that  there  are  really  as  many  pro- 
toplasts, as  we  have  seen  a  nucleus 
is  an  essential  part  of  one  of  the 
latter.  Moreover,  a  single  proto- 
plast contains  only  a  single  nucleus. 

The  difference  between  a  colony 
of  this  kind  and  one  constructed 
like  Chroococcus  or  Volvox  is  the 
absence  of  a  cell-wall  between  the 
protoplasts.  They  are  a  stage 
higher  than  the  Myxomycetes,  as 
the  whole  colony  is  protected  by  an 
external  membrane. 

Other  ccenocytes  exist  in  which, 
besides  the  limiting  wall,  certain 
transverse  walls  exist,  dividing  up 
the  chamber  into  compartments. 
This  condition  is  intermediate 

between  the  coenocyte  already  described  and  the  simple 
colony  or  the  multicellular  plant. 

In  most  cases  the  division  of  the  cells  goes  on  for  a  con- 
siderable time  and  may  continue  almost  indefinitely,  the 
number  of  the  constituent  protoplasts  becoming  very  great 
and  the  colony  proportionately  large.  According  to  the 


FIG.  12. — EMBRYO  OF  Orobus  AT 
THE  BASE  OF  A  LONG  Sus- 
PENSOR.  THE  LATTER  SHOWS 
A  CCENOCYTIC  STRUCTURE. 
(After  Guignard.) 


12  VEGETABLE  PHYSIOLOGY 

direction  of  the  divisions  we  get  filaments  (fig.  13),  plates 
(fig.  14),  or  masses  of  cells,  the  latter  undergoing  much 
subsequent  differentiation  according  to  their  ultimate 
dimensions  and  the  nature  of  their  habitat  or  environment. 


FIG.  13. — FILAMENTS  OF  Nottoc. 
(After  Luerssen.) 


FIG.  14. — Fed  aslrum,  CONSIST- 
ING OF  A  PLATE  OF  CELLS. 


The  protoplasm  being  the  living  substance  of  the  plant 
is  possessed  of  certain  properties  which  are  not  shared  by 
the  framework  on  which  it  rests.  It  is,  indeed,  the  centre 
of  all  the  activities  which  the  plant  manifests.  It  assimi- 
lates the  food  which  the  plant  requires  and  carries  out  all 


FIG.  15. — VEGETABLE  CELLS. 

A,  very  young ;    B,  a  little  older,  showing  commencing  formation  of  vacuole. 
p,  protoplasm  ;   n,  nucleus  ;   v,  a  vacuole. 

the  chemical  processes  necessary  for  life.  It  constructs 
the  framework  of  the  plant  by  which  it  is  itself  supported. 
It  receives  impressions  from  without,  and  regulates  the 
responses  which  the  plant  as  a  whole  makes  to  those  im- 
pressions, both  by  internal  and  external  movements  or 
changes  of  position,  and  by  modification  of  its  metabolism. 


THE  GENERAL  STRUCTURE  OF  PLANTS      13 


It  is  only  by  its  powers  of  responding  to  such  impressions 
that  the  whole  organism  is  able  to  place  itself  in  harmony 
with  its  environment.  Finally,  it  carries  out  the  processes 
of  reproduction. 

The  primary  needs  of  a  plant  are  fairly  simple.  If  we 
study  the  life  and  the  behaviour  of  one  of  the  free-swim- 
ming organisms  of  which  we  have  already  spoken,  we  see 
that  its  first  requirement  is  water.  In  this  it  lives  ;  from 
this  it  draws  its  supplies  of  nutriment  and  into  this  it  pours 
forth  its  excreta.  The  arrangement  of  the  protoplasm  in 
the  cell  in  one  of  the  higher  plants  points  to  a  similar  need. 
If  we  regard  the  arrangement  whether  in  the  young  or  the 
adult  cell,  we  notice  particularly  the  very  close  relation  of 
the  protoplasm  to  water.  The  young  cell  enclosed  in  its  cell- 
membrane  speedily  shows  a  tendency 
to  accumulate  water  in  its  interior, 
and  gradually  drops  appear  in  its  sub- 
stance which  lead  ultimately  to  the 
formation  of  a  vacuole  always  full  of 
liquid  (figs.  15,  16).  This  store  of 
water  in  the  interior  of  a  cell  is  of 
almost  universal  occurrence  in  the 
lowly  as  well  as  the  highly  organised 
plant.  The  constitution  of  proto- 
plasm, so  far  as  we  know  it,  depends 
upon  this  relation,  for  the  appa- 
rently structureless  substance  is 
always  saturated  with  water.  It  is  only 
while  in  such  a  condition  that  a  cell 
can  live  ;  with  very  rare  exceptions, 
if  a  cell  is  once  completely  dried, 

even  at  a  low  temperature,  its  life  is  gone,  and  restoration 
of  water  fails  to  enable  it  to  recover. 

The  constancy  of  the  occurrence  of  the  vacuole  in  the 
cells  of  the  vegetable  organism  is  itself  an  evidence  that 
such  cells  are  completely  dependent  upon  water  for  the 
maintenance  of  life.  The  cell-wall,  though  usually 


FIQ.  16. — ADULT  VEGETABLE 
CELLS.  x  500.  (After 
Sachs.) 

h,  cell- wall;  p,  protoplasm; 
k  k',  nucleus,  with  nuoleoli  j 
s'  s,  vacuoles. 


14  VEGETABLE  PHYSIOLOGY 

permeable,  yet  presents  a  certain  obstacle  to  the  absorption 
of  water,  and  so  even  those  cells  which  are  living  in 
streams  or  ponds  usually  possess  a  vacuole.  Cells  without 
a  membrane,  such  as  the  zoospores,  already  many  times 
mentioned,  can  more  readily  absorb  water  from  without, 
and  hence  they  are  not  vacuolated  to  the  same  extent  as 
the  former  ones  ;  indeed,  many  of  them  have  no  vacuoles. 
Where  the  vacuole  exists  it  always  contains  water,  so 
that  the  protoplasm  of  the  cell  has  ready  access  to  it,  as 
much  so  indeed  as  the  cell  which  possesses  no  wall.  The 
vacuole  contains  a  store  which  is  always  available. 

The  advantages  which  water  supplies  to  the  plants  are 
many.  In  the  first  place,  we  have  seen  there  is  a  very  close 
connection  between  it  and  the  protoplasm,  the  life  of  the 
latter  being  dependent  upon  its  presence.  The  information 
we  have  at  present  does  not  enable  us  to  explain  the  nature 
of  this  dependence.  There  are  other  features  of  the  rela- 
tionship, howev er,  into  which  we  can  enter  more  fully.  The 
protoplasm  derives  its  food  from  substances  in  solution  in 
the  water  ;  the  various  waste  products  which  are  incident 
to  its  life  are  excreted  into  it  and  so  removed  from  the 
sphere  of  its  activity.  The  raw  materials  from  which  cer- 
tain cells  construct  the  food  which  is  ultimately  assimilated 
are  absorbed  from  the  exterior  in  solution  in  water.  More- 
over, water  is  the  ultimate  medium  through  which  gaseous 
constituents  necessary  for  life  reach  the  protoplasm. 

Passing  from  the  consideration  of  the  protoplasm  in 
particular,  the  plant  as  a  whole  shows  a  similar  dependence 
on  water.  Many  parts  owe  their  rigidity  to  the  distension 
of  their  cells  by  liquid  ;  growth  of  the  different  members  is 
dependent  upon  the  same  hydrostatic  pressure.  In  many 
cases  communication  between  different  parts  of  a  plant  is 
brought  about  through  the  same  instrumentality,  and  thus 
the  response  of  the  plant  to  various  forms  of  stimulation 
is  facilitated  or  indeed  made  possible. 

Another  primal  necessity  of  the  plant  is  air.  Every 
living  organism,  with  the  exception  of  a  few  of  the  very 


THE  GENEEAL  STKUCTUKE  OF  PLANTS      15 

lowly  forms  of  microbes,  is  dependent  on  the  access  of  oxygen 
for  the  maintenance  of  life. 

The  oxygen  is  usually  obtained  by  the  plant  through 
the  intervention  of  water.  The  aquatic  plant,  whether 
free-swimming  or  stationary,  unicellular  or  possessed  of  a 
highly  differentiated  body,  absorbs  the  needed  supply  from 
the  quantity  which  is  dissolved  in  the  water  of  the  sea, 
stream,  or  pool  in  which  it  lives.  The  terrestrial  plant  conveys 
it  to  the  protoplasts  in  solution  in  the  water  with  which  its 
tissues  or  its  walls  are  saturated.  In  such  an  organism 
there  is,  however,  need  of  a  special  mechanism  by  means 
of  which  the  gases  of  the  exterior  may  obtain  access  to 
the  living  cells  in  the  interior  of  the  mass. 

A  third  requirement  of  the  plant  is  food.  Here 
ultimately,  again,  its  dependence  is  placed  upon  the  water 
it  obtains.  The  food  or  the  materials  from  which  the  food 
is  constructed  are  absorbed  by  the  plant  in  solution  in 
water,  whether  the  food  material  is  solid,  liquid,  or  gaseous 
in  the  condition  in  which  it  is  presented  to  it. 

Another  condition  is  imperative  in  the  case  of  a  plant 
which  is  composed  of  a  large  number  of  protoplasts  or  cells. 
Not  only  must  each  have  its  own  needs  supplied,  but  it 
must  be  in  a  condition  to  influence  others  and  be  influenced 
by  them.  In  such  a  plant  we  have,  in  fact,  a  community  of 
individuals,  situated  differently  with  regard  to  the  supply 
of  individual  and  collective  needs,  and  the  well-being  of 
the  whole  community  must  depend  upon  the  co-operation 
of  all  in  carrying  out  the  different  processes  of  life.  The 
protoplasts  of  such  a  community  must  therefore  be  in 
organic  connection  with  each  other,  so  that  such  co-opera- 
tion can  be  secured.  The  connection  between  contiguous 
protoplasts  which  are  separated  by  cell-walls  is  not  easy 
to  determine.  Special  methods  of  preparation,  and  the 
application  of  particular  staining  reagents,  will  show,  how- 
ever, under  very  high  magnification,  that  the  living  sub- 
stance of  one  cell  is  continuous  with  that  of  its  neighbour 
by  fine  delicate  fibrils  which  perforate  the  wall  (fig.  17). 


16 


VEGETABLE  PHYSIOLOGY 


In  a  few  cases,  as  in  certain  seaweeds,  and  in  the  sieve- 
tubes  of  the  flowering  plants,  the  connecting  strands  are 
sufficiently  coarse  to  be  visible  under  a  comparatively  low 
power  of  the  microscope,  and  to  need  hardly  any  special 
preparation  (fig.  18). 

It  will  no  doubt  have  been  noticed  that  the  term  '  cell ' 
is  somewhat  loosely  used.     A  typical  cell  of  a  multicellular 


Fio.  17. — CONTINUITY  OF  THE  PROTOPLASM  OF 
CONTIGUOUS  CELLS  OF  THE  ENDOSPERM 
OF  A  PALM  SEED  (Bentinckia).  Highly 
magnified.  (After  Gardiner.) 

a,  contracted  protoplasm  of  a  cell ;  b,  a 
group  of  delicate  protoplasmic  filaments 
passing  through  a  pit  in  the  cell-wall. 


FIG.  18.— SEMI-DIAGRAMMATIC  LON- 
GITUDINAL SECTION  OF  AN  OLD  AND 
STOUT  PORTION  OF  Ceramium  ru- 
brum,  SHOWING  CONTINUITY  BE- 
TWEEN THE  PROTOPLASMIC  CON- 
TENTS OF  THE  AXIAL  OR  CENTRAL 
CELLS,  a,  a,  AT  THEIR  ENDS,  AND 

LATERALLY     WITH     THE     CORTICAL 

CELLS  b,  BY    MEANS    OF  PROTO- 
PLASMIC THREADS.     (After  Hick.) 


plant  consists  of  three  parts — the  protoplast,  the  cell- wall, 
and  the  vacuole  (fig.  6) ;  of  these  the  first  is  the  most 
important,  being  the  living  substance.  A  protoplast  which 
has  no  cell-wall  and  contains  no  vacuole  is  still  called  a  cell. 
The  term  is  again  often  applied  to  a  cavity  which  contains 
no  protoplast,  as  in  the  case  of  old  wood  or  cork.  In  such 
cases  a  protoplast  once  occupied  the  cavity,  but  it  has  been 
removed  by  death.  These  cells  or  cavities  are  consequently 
only  the  skeletons  of  dead  protoplasts. 


17 


CHAPTEK  II 

THE    DIFFERENTIATION    OF    THE    PLANT-BODY 

The  primary  needs  of  a  complex  plant  are  the  same  as  those 
of  a  single  protoplast,  the  greater  size  of  the  former  involv- 
ing, however,  a  more  elaborate  method  of  supplying  them. 
In  multicelmlar  plants  we  consequently  meet  with  a  con- 
siderable degree  of  differentiation  of  structure.  Each  proto- 
plast, which  is  one  of  the  units  of  the  colony,  has  originally 
the  same  properties  as  the  unicellular  plant.  With  increase 
of  number  in  the  plant-body,  and  with  the  consequent 
increase  of  size,  a  certain  division  of  labour  soon  makes 
its  appearance,  and  particular  groups  of  cells  develop  one  of 
these  properties  more  than  the  others.  A  specialisation  of 
powers  is  very  quickly  apparent,  and  we  can  recognise  masses 
of  cells  devoted  to  the  discharge  of  one  function,  others  to 
that  of  another,  and  so  on.  Such  limitations  of  the  powers 
and  properties  of  the  individuals  have  for  their  object  the 
well-being  of  the  community  of  which  those  individuals 
are  constituents. 

Various  groups  of  plants  show  this  specialisation  of 
function  or  differentiation  of  structure  in  very  different 
degrees,  any  particular  development  having  a  special 
reference  to  the  habitat  or  the  mode  of  life  which  is 
characteristic  of  the  community  in  question.  A  plant-body 
which  takes  the  form  of  a  long  filament  or  a  plate  of  cells 
shows  little  differentiation  beyond  the  formation  of  a 
vacuole  in  each  protoplast. 

The  setting  apart  of  special  cells  for  purposes  of  repro- 
duction is  generally  the  first  specialisation  which  takes  place. 

2 


18  VEGETABLE  PHYSIOLOGY 

As  soon  as  the  cells  of  the  plant  begin  to  divide  in 
three  dimensions,  so  that  a  mass  of  protoplasts  is  formed, 
the  progress  of  differentiation  becomes  marked. 

In  such  a  mass  the  necessity  of  supplying  water  to  all 
the  constituent  units  involves  particular  difficulties  which 
vary  according  to  the  environment  of  the  plant  under 
observation.  Those  which  live  in  water  need  much  less 
complex  arrangements  than  those  which  are  at  home  on 
land,  as  they  can  absorb  water  from  the  exterior  by  their 
general  surface,  and  after  absorption  it  can  easily  make  its 
way  from  cell  to  cell.  Those  which  derive  their  supply  of 
water  entirely  from  the  soil,  as  is  the  case  with  nearly  all 
terrestrial  plants,  need  a  specialised  mechanism  for  trans- 
port of  the  water  after  it  has  been  taken  up. 

On  the  other  hand,  the  supply  of  a  suitable  atmosphere 
to  the  interior  of  the  plant  for  the  service  of  its  more 
deeply  seated  protoplasts  is  attended  with  more  difficulty 
in  the  case  of  an  aquatic  than  a  terrestrial  plant. 

In  cell-masses,  therefore,  such  as  are  found  in  all  plants 
possessing  more  than  microscopic  dimensions,  we  meet 
with  considerable  differentiation  of  the  plant-body,  both 
in  form  and  structure.  The  explanation  of  the  details 
of  such  differentiation  is  to  be  found  in  the  division  of 
labour  which  the  size  and  the  mode  of  life  of  the  particular 
plant  demand. 

In  ah1  the  higher  plants  the  cell-mass  or  body  of  the 
plant  can  be  seen  to  possess  a  subterranean  portion — the 
root  system — and  a  subaerial  portion — the  shoot,  each  of 
which  has  its  own  functions  to  discharge,  and  is  exposed  to 
particular  dangers  against  which  it  needs  protection. 

The  first  advantage  secured  for  the  plant  by  its  root 
system  is  a  firm  anchorage  to  the  soil.  This  is  not  secured 
without  difficulty  and  even  danger,  for  to  become  fixed 
in  the  soil  the  root  must  penetrate  it,  a  process  which  it 
can  only  carry  oufc  by  the  slow  method  of  gradual  growth. 
The  composition  of  the  soil  offers  certain  difficulties  to  this 
penetration ;  it  may  be  too  dense  or  too  powdery,  too  dry 


THE  DIFFERENTIATION  OF  THE  PLANT-BODY   19 

or  too  wet ;  it  may  be  slimy  like  clay,  or  very  stony  like 
gravel.  The  amount  of  water  the  soil  contains  and  the 
degree  in  which  air  is  present  are  also  factors  which  must 
be  taken  into  account  in  considering  the  growth.  After 
a  plant  has  once  established  itself  and  secured  firm  anchorage 
it  still  has  to  deal  with  varying  conditions  of  a  similar 
nature,  for  the  character  of  the  soil  is  very  liable  to  changes, 
depending  on  conditions  of  temperature,  weather,  and 
so  on. 

Besides  the  advantage  of  a  firm  anchorage,  the  root 
depends  upon  the  soil  for  the  supply  of  certain  materials 
which  ultimately  take  part  in  some  way  in  its  nutritive 
processes.  Certain  minerals  are  necessary  to  every  green 
plant,  many  others  are  advantageous,  some  are  deleterious. 

The  copious  branching  which  the  root  system  exhibits 
goes  a  long  way  to  secure  good  anchorage,  and  at  the  same 
time  to  draw  supplies  from  a  large  bulk  of  soil.  The 
branching  is  supplemented  by  the  development  of  the 
root  hairs,  long  unicellular  outgrowths  of  the  surface  of 
each  young  branch  of  the  root,  which  come  into  very  close 
contact  with  the  ultimate  particles  of  the  soil. 

If  we  turn  to  inquire  what  dangers  beset  the  part  of 
the  plant  we  have  called  the  shoot,  which  grows  up  into 
the  air  and  forms  a  head  that  is  frequently  of  large  size, 
we  find  them  associated  with  the  various  atmospheric 
changes  incident  to  every  climate.  First  of  these  we  may 
place  wind  or  tempest.  As  the  shoot-body  grows  it  must 
offer  more  and  more  resistance  to  air  currents,  a  resistance 
which  may  easily  result  in  a  violent  uprooting  of  the  plant. 
This  involves  such  a  subdivision  of  the  plant-body  as  will 
allow  the  wind  to  penetrate  through  it  without  serious 
disturbance.  We  see  consequently  a  continuous  tapering 
off  of  the  branches  and  twigs,  which  become  more  and 
more  flexible  as  they  are  increasingly  slender.  In  the 
central  part  of  the  shoot  system  they  are  rigid  and  can 
resist  a  storm  ;  where  by  their  dimensions  resistance  becomes 
impracticable  we  find  flexibility,  enabling  them  to  bow 

2  * 


20  VEGETABLE  PHYSIOLOGY 

to  the  wind,  often  so  completely  as  to  place  their  long  axes 
parallel  to  the  direction  in  which  it  is  blowing. 

Another  reason  for  this  continued  subdivision  of  the 
plant-body  is  found  in  its  relation  to  the  absorption  from 
the  soil  which  we  have  found  associated  with  the  root. 
The  latter  is  continually  absorbing  the  water  of  the  soil ; 
after  separating  from  such  water  the  mineral  constituents 
it  contains,  a  very  large  part  of  the  water  is  evaporated, 
and  so  passes  to  the  exterior  again.  To  favour  such  evapora- 
tion it  is  advantageous  that  the  ratio  between  surface  and 
bulk  shall  be  a  large  one,  and  so  the  great  subdivision  of 
the  subaerial  part  of  the  plant  is  concerned  in  solving 
the  problem  of  its  nourishment. 

Indirectly  the  composition  of  the  subaerial  part  of 
the  plant  has  an  application  to  a  danger  to  which  the 
underground  region  is  exposed.  The  pressure  of  the  wind 
upon  an  unyielding  surface  in  the  air  would  be  attended 
by  great  danger  to  the  anchoring  root,  which  might  be 
violently  pulled  from  the  ground  by  the  leverage  exerted 
by  such  pressure.  The  great  subdivision  of  the  shoot 
system  and  the  flexibility  of  its  ultimate  twigs  minimises 
this  danger,  but  even  as  it  is  it  is  not  unusual  after  a  tempest 
to  notice  the  uprooting  of  trees  of  quite  considerable  girth. 

The  distribution  of  the  water  of  rainstorms  calls  for 
particular  arrangements  of  the  parts  of  the  shoot.  The 
water  can  be  led  either  towards  or  away  from  the  centre 
of  the  plant.  Should  the  root  system  be  one  which  spreads 
considerably  and  extends  to  long  distances  below  the 
surface  of  the  soil,  it  is  of  great  importance  that  the  rainfall 
collected  on  the  central  mass  of  the  shoot  system  shall  be 
distributed  widely  so  as  to  reach  the  extremities  of  the 
roots,  watering  thus  a  large  area  of  ground.  This  is  brought 
about  by  suitable  positions  taken  by  the  flattened  parts  and 
the  grooving  of  certain  of  the  cylindrical  parts  of  the  shoot 
system,  causing  the  water  to  be  conducted  outwards.  If 
the  root  system  consists,  on  the  other  hand,  of  a  strong  main 
root  with  comparatively  few  branches  this  arrangement 


THE  DIFFERENTIATION  OF  THE  PLANT-BODY    21 

would  largely  deprive  it  of  water.  Hence  in  plants  with 
roots  distributed  in  this  way  we  find  devices  to  conduct 
the  water  into  the  centre  of  the  mass  of  the  shoot  system. 

When  we  pass  to  a  closer  examination  of  the  much 
divided  or  branched  shoot  we  almost  invariably  find  that  its 
ultimate  twigs  put  forth  certain  regularly  arranged  flattened 
expansions.  In  cases  where  there  is  much  exposure  to 
currents  of  air  these  flattened  portions  are  furnished  with 
stalks  of  variable  length,  which  are  extremely  flexible  and 
allow  the  flattened  organs  to  sway  freely  backwards  and 
forwards  as  the  wind  blows  upon  them.  These  flattened 
portions  further  are  usually  of  a  vivid  green  colour.  They 
are  known  as  leaves,  or  preferably  foliage  leaves. 

As  almost  all  plants  possess  leaves,  we  may  inquire 
why  these  organs  should  so  uniformly  be  thin  and  flat. 

There  are  several  reasons  of  almost  equal  importance. 
The  leaf  or  other  winged  part  of  the  shoot  is  in  contact  or 
relation  with  the  air  only.  Interchanges  of  gases  between 
the  air  and  the  leaf  are  continually  going  on,  and  these 
interchanges  are  effected  most  easily  and  fully  by  means 
of  a  large  extent  of  surface.  No  form  gives  so  much  surface 
in  proportion  to  its  bulk  as  a  thin  flat  plate,  just  such  a 
form  indeed  as  the  flattened  portion  or  blade  of  the  leaf. 
The  interchanges  include  the  absorption  of  particular  gases 
from  the  air,  and  the  giving  out  of  gases  and  water  vapour. 
As  we  shall  see  later,  the  internal  structure  of  the  leaf -blade 
is  arranged  largely  with  a  view  to  the  carrying  out  of  these 
exchanges. 

A  second  reason  for  the  flattening  of  the  leaf  is  concerned 
with  the  manufacture  of  the  plant's  food.  A  particular 
gas  known  as  carbon  dioxide,  which  is  taken  in  from  the 
air,  is  ultimately  built  up  into  a  true  food,  a  kind  of  sugar. 
Though  the  formation  of  sugar  in  the  plant  is  only  partly 
understood,  it  is  known  to  depend  upon  the  presence  of 
the  green  colouring  matter  and  upon  its  being  properly 
illuminated.  The  flattened  form  helps  to  expose  the  green 
pigment  to  the  light  to  the  greatest  advantage. 


22  VEGETABLE  PHYSIOLOGY 

Yet  a  third  reason  may  be  given.  The  leaves  are  very 
frequently  so  placed  that  they  extend  outwards  from 
the  plant  and  lie  nearly  parallel  to  the  surface  of  the  ground. 
In  this  way  they  present  their  edges  to  the  wind  and  offer 
as  little  obstacle  as  possible  to  its  passage  through  the 
tree,  so  minimising  the  risk  of  being  torn  off  when  the  force 
of  the  wind  is  strong.  As  the  wind  passes  between  them 
they  are  made  to  rise  and  fall,  but  they  offer  much  less 
resistance  to  its  force  than  they  would  if  they  were  not 
flattened. 

Besides  the  plants  which  we  have  been  examining  there 
are  other  forms  of  terrestrial  habits  which  possess  only 
weak  axes,  quite  incapable  of  supporting  any  great 
development  of  their  shoot  system.  These  obtain  support 
by  clinging  in  various  ways  and  holding  by  various  mechan- 
isms to  other  structures,  such  as  the  trunks  of  trees,  walls, 
&c.  In  some  cases  they  develop  accessory  root  systems 
from  some  part  of  their  shoots,  such  roots,  usually  of  small 
dimensions,  penetrating  their  supports  and  so  securing 
anchorage. 

The  first  indication  of  structural  differentiation  in  the 
vegetative  body  of  the  plant  is  a  change  in  the  character 
of  the  exterior,  which  has  for  its 
object  the  protection  of  the  plant 
from  external  injurious  influences. 
This  can  be  seen  even  among  the 
seaweeds,  simple  as  is  generally  the 
structure  of  members  of  this  group. 
Fucus  and  its  allies,  which  form  part 

of  the  class  of  the  brown  A1g*>  have 
SHOWING  CHARACTER  OF   their    external    cells    much    smaller, 

more  closely  put  together'  and 

generally  much  denser  than  the  rest 
of  their  tissue  (fig.  19).     In  the  group 
of  the  Mosses  certain  arrangements  of  this   kind  can  be 
seen.     The  common  bog  moss  (Sphagnum)  shows  its  stem 
to  have  on  the  outside  several  layers  of  large  empty  cells 


THE  DIFFEKENTIATION  OF  THE  PLANT-BODY    23 
whose  walls   are  marked  with  spiral   thickenings.     Inside 


Fro.  20. — TRANSVERSE  SECTION  OF 
STEM  OB1  Sphagnum. 


FIG.  21.— SECTION  OF  STEM  OF  Moss, 
SHOWING  CENTRAL  STRAND  OF 
THIN-WALLED  CELLS  SURROUNDED 
BY  CORTEX  AND  EPIDERMIS.  THE 
WALLS  OF  THE  OUTER  CELLS  OF 
THE  CORTEX  ARE  CONSIDERABLY 
THICKENED.  (After  Sachs.) 


these  a   further   protective  layer  of  small  cells  with  uni- 
formly thick  walls  is  met  with  (fig.  20).     In  the  smaller 


Fia.  22. — TRANSVERSE  SECTION  OF  THE  BLADE  OF  A  LEAF,  SHOWING  THE 
OUTER  WALLS  OF  THE  EPIDERMAL  CELLS  THICKENED  AND  CUTICULARISED. 
X  100. 

mosses    the    outer    layers    of    the    cortex   are   thickened 
(fig.  21). 
In   the   higher   terrestrial  plants  we  have  evidence  of 


24 


VEGETABLE  PHYSIOLOGY 


great  specialisation  for  protective  purposes,  a  particular  tegu- 
mentary  system  being  developed,  which  varies  in  com- 
plexity in  the  different  groups.  In  the  smallest  forms, 

which  are  only  herbaceous  in 
habit,  we  find  the  protective 
mechanism  taking  the  shape  of 
a  thickening  and  cuticularisa- 
tion  of  the  outer  walls  of  the 

FIG.  23. — OUTER  PORTION  OF   CORTEX          ,,         „     .,  ,     , 

OF  YOUNG  TWIG  OF  LIME.  G^  °f   the   outermost   layer 

per,  cork  layer.       ph,  meristem  layer.      (fig-      22).          The       protection 

secured  is  twofold  :  evapora- 
tion of  water  is  prevented,  and  so  an  economy  of  the  supply 
is  secured,  while  the  dangers  incident  to  cold  or  heat  are 
minimised. 


~~pe 


FIG.  24. — SECTION  OF  BARK  OF  Quercus  sessili flora.     (After  Kny.) 
pe,  cork  layers  arising  at  different  depths  in  the  cortex. 

In  plants  of  sturdier  habit  the  protection  afforded  by 
this  outermost  layer  or  epidermis  is  supplemented  after  a 
while  by  the  development  of  a  more  complicated  tegu- 
mentary  sheath  which  replaces  the  epidermis  when  the 


THE  DIFFEKENTIATION  OF  THE  PLANT-BODY    25 


FlQ.     26.— COLLENCHYMA     TINDER 

THE    EPIDERMIS    OP    PETIOLE. 
X  50. 


latter  is  worn  away.  Certain  cells  become  specialised  and 
form  layers  of  cork  (fig.  23),  which  arise  successively  at 
gradually  increasing  distances  from  the  exterior,  and  in  the 
case  of  trees  finally  lead  to  the  construction  of  a  bark  (fig.  24). 
The  corky  formations  are  supple- 
mented by  masses  or  sheaths  of 
hardened  or  sclerenchymatous 
parenchyma,  or  even  by  scleren- 
chyma  itself.  In  forms  which 
are  intermediate  in  requirements, 
such  as  the  petioles  of  leaves, 
layers  of  collenchyma  are  de- 
veloped below  the  epidermis 
(fig.  25). 

Sometimes  sheaths  or  layers 
of  sclerenchyma  are  developed 
instead  of  cork  ;  this  condition 
occurs  especially  among  the  stouter  Monocotyledons. 

The  protective  mechanisms  developed  by  roots  also  show 
a  good  deal  of  variety.  The  outermost  layer  does  not  at  first 
take  the  form  of  an  impervious  membrane ;  this  would  be 
inconvenient  in  view  of  the  necessity  for  the  existence  of 
root-hairs.  In  some  cases  the  second 
layer  undergoes  modification,  its  cells 
fit  closely  together,  and  the  radial 
walls  become  cuticularised  where  they 
are  in  contact  with  each  other  (fig. 
26,  ex)  ;  it  then  constitutes  the 
exodermis.  Later  the  corky  change 
extends  to  all  the  cell-walls  of  this 

layer.  Other  sheathing  layers  are  also  found  more  deeply 
seated,  while  eventually  the  pericycle  becomes  the  place  of 
formation  of  corky  tissue. 

The  second  prominent  differentiation  which  presents  itself 
is  the  formation  of  a  system  of  cells  and  vessels  for  the  trans- 
port of  water  through  the  plant  and  the  circulation  of  nutri- 
tive and  other  materials.  We  may  speak  of  this  as  the 


Fia.26. — SECTION  OF  OUTER 
REGION  OF  ROOT,  SHOW- 
ING EXODERMIS,  ex. 


26  VEGETABLE  PHYSIOLOGY 

conducting  system.  A  little  reflection  will  show  us  the  neces- 
sity for  the  development  of  some  such  system  as  this,  which 
must  be  more  extensive  and  complex  as  the  size  of  the 
plant  increases.  We  find  that  the  source  of  water  on  which 
a  terrestrial  plant  depends  is  the  soil  in  which  its  roots  are 
embedded.  Even  when  it  is  young  many  of  its  protoplasts 
are  placed  at  a  considerable  distance  from  such  a  source  of 
supply,  and  in  the  absence  of  a  ready  means  of  communica- 
tion must  die  in  consequence  of  their  position.  These, 
moreover,  are  among  the  most  active  of  the  protoplasts 
discharging  important  duties  in  connection  with  nutrition, 
and  needing  for  their  purpose  considerable  quantities  of 
the  water  from  the  soil  with  certain  salts  dissolved  in  it. 
Much  of  this  water  must  be  evaporated  to  enable  continuous 
absorption  to  take  place. 

The  main  conducting  system  is  formed  by  the  collections 
of  cells  and  vessels  which  are  known  as  the  vascular  bundles. 
These  structures  consist  in  most  cases  of  two  parts,  the 
wood,  which  is  the  path  for  the  ascent  of  water  from  the  roots, 
and  the  last,  which  is  more  concerned  with  the  transport 
of  the  elaborated  products  of  the  metabolism  of  the 
cells. 

The  degree  of  development  of  this  system  varies  very 
much  in  different  plants.  In  an  ordinary  herbaceous 
Dicotyledon  the  bundles  remain  separate,  and  can  be 
traced  separately  from  the  root,  through  the  stem  to  the 
leaves  (fig.  27)  in  which  they  form  the  branching  network 
known  as  the  veins  (fig.  28).  With  greater  size,  however, 
more  capacious  channels  are  demanded,  and  we  find  more 
and  more  bundles  developed,  until  we  reach  the  condition 
of  the  oldest  trees,  nearly  the  whole  of  whose  trunks  are 
formed  of  tissue  which  either  is  or  has  been  devoted  to  this 
service.  In  such  trees  the  most  actively  living  parts  are 
found  at  the  extremities,  by  far  the  greatest  number  of 
their  protoplasts  being  situated  in  the  twigs  and  leaves. 
Indeed,  the  greater  part  of  the  wood  of  the  trunk  of  many 
trees  is  dead,  and  consequently  functionless. 


THE  DIPFEEENTIATION  OF  THE  PLANT-BODY    27 


The  same  tissues  serve  for  transport  in  the  Monocotyle- 
dons, and  in  the  Vascular  Cryptogams,  though  the  mode  of 
arrangement  of  the  elements  is  altogether  different  from 
that  of  the  Dicotyledons. 

In  those  vascular  plants  which  live  in  water,  and  parti- 
cularly in  those  which  are  totally  submerged,  there  is  no 

need  for  so  elaborate  a  transport 
system,  as  water  can  be  readily 
absorbed  by  the  general  surface. 
We  find  two  modifications  of 
structure  in  such  plants ;  the 
epidermis  is  hardly  at  all  cuti- 


FIG.  27.— DIAGRAM  OF  THE  COURSE 
OF  THE  VASCULAR  BUNDLES  IN 
AN  HERBACEOUS  DIOOTYLEDO- 
NOUS  PLANT. 


FIG.  28. — DISTRIBUTION  OF  THE 
VASCULAR  BUNDLES  OR  VEINS 
IN  A  FOLIAGE  LEAP. 


cularised,  so  that  water  can  pass  from  the  exterior  into  its 
cells  ;  while  the  vascular  bundles  are  comparatively  feebly 
developed,  the  woody  part  of  them  being  particularly  small. 
The  third  requirement  of  a  plant  of  considerable  mass, 
especially  if  it  has  a  terrestrial  habitat,  we  have  seen  to  be 
a  power  of  resisting  such  external  forces  as  would  lead  to 
its  uprooting,  which  must  be  combined  with  a  considerable 
degree  of  flexibility,  at  any  rate  at  the  extremities  of  the 
body.  This  combination  of  rigidity  and  flexibility  has  been 
secured  in  various  ways,  varieties  of  both  the  form  and 
the  structure  of  the  plant  being  concerned  in  it.  In  the 
simplest  plants  but  little  differentiation  of  the  body  is 


28  VEGETABLE  PHYSIOLOGY 

needed ;  such  forms  as  consist  of  single  cells,  or  rows  or 
plates  of  cells,  living  in  water,  need  hardly  any  rigidity, 
and  in  their  cases  the  unthickened  cell- wall  affords  sufficient 
support  to  the  protoplasm.  Larger  plants  which  grow  in 
rapidly  flowing  water  usually  possess  flexible  stems  and 
much-divided  leaves,  which  consequently  give  way  to  the 
current  and  escape  damage.  Small  terrestrial  plants  or 
parts  of  plants,  which  have  but  a  short  life,  resemble  these 
aquatic  forms  in  their  general  characteristics,  though  they 
show  much  greater  variety  in  the  forms  of  their  leaves. 
The  rigidity  and  flexibility  of  both  depend  upon  the  disten- 
sion of  their  cells  with  water.  We  find  this  mechanism  in 
succulent  petioles,  such  as  those  of  the  rhubarb,  and  in 
certain  herbaceous  stems  which  contain  little  wood,  such 
as  those  of  the  cabbage  and  lettuce.  Plants  of  terres- 
trial habit  which  attain  very  large  dimensions,  such  as  the 
forest  trees,  need,  however,  much  greater  modification.  We 
have  already  studied  how  the  outward  form  of  their 
different  parts,  the  root  and  the  shoot,  is  adapted  to  their 
respective  situations.  Their  internal  structure  also  corre- 
sponds to  their  requirements,  and  helps  to  secure  their  safety. 
In  many  cases  the  strength  and  prominence  of  the 
tegumentary  and  conducting  tissues  supply  the  particular 
need  of  each  part.  In  most  forest  trees  we  have  seen  that 
anchorage  is  afforded  by  the  strong  much-branched  root 
system,  the  centre  of  whose  members  is  composed  of  great 
developments  of  secondary  wood,  forming  part  of  the 
conducting  system.  The  trunk  and  twigs  are  of  similar 
composition,  the  former  being  strengthened  also  by  its  bark. 
The  distribution  of  the  hard  woody  conducting  tissue  is 
very  different  in  the  subterranean  and  subaerial  portions 
of  the  axis.  In  the  former  it  is  found  to  take  the  shape  of 
a  solid  central  core,  a  form  which  is  well  adapted  to  resist  an 
uprooting  pull,  while  it  allows  a  considerable  bending.  In 
the  subaerial  portion  as  soon  as  the  woody  portion  is 
developed  it  is  found  to  be  composed  of  several  tough 
strands  which,  while  the  plant  is  young,  are  separate  from 


THE  DIFFERENTIATION  OF  THE  PLANT-BODY    29 

each  other  and  are  disposed  in  a  ring  comparatively  near 
the  periphery.  They  thus  afford  an  effective  resistance 
to  such  a  lateral  strain  as  they  would  be  subjected  to  during 
a  high  wind.  This  is  very  conspicuous  in  hollow  herbaceous 
stalks.  Later  on,  if  the  shoot  increases  in  dimensions,  the 
ring  of  separate  strands  is  replaced  by  a  central  core  not 
unlike  that  of  the  root. 

The  distribution  of  the  woody  elements  in  the  leaf 
is  different  from  either.  They  exist  in  the  form  of  the 
so-called  veins,  which  constitute  a  network  of  very  tough 
fibrous  bands  upon  which  the  delicate  tissue,  easily  tearable, 
is  supported.  These  strands  usually  strengthen  particularly 
the  margins  and  apex  of  the  leaf  blade  and  protect  it  from 
being  torn.  The  blade,  therefore,  when  acted  on  by  wind  is 
made  to  play  as'  a  single  rigid  piece  moving  up  and  down 
without  losing  its  flatness  for  a  moment. 

There  are,  however,  many  herbaceous  forms,  whose  re- 
quirements are  similar  to  those  of  the  very  young  axis  of 
the  woody  plant,  but  which  have  not  a  very  great  develop- 
ment of  either  primary  tegumentary  tissue  or  of  vascular 
bundles.  With  no  additional  mechanism  for  support,  they 
would  be  in  great  danger  of  either  collapsing  or  being  actually 
uprooted.  In  their  cases  we  meet  with  a  subsidiary  develop- 
ment of  supporting  tissue,  which  shows  a  great  variety  in 
its  arrangement  and  distribution. 

We  find  that  the  tissue  which  most  frequently  subserves 
this  purpose  is  either  collenchyma,  sclerenchymatous  par- 
enchyma, or  true  sclerenchyma.  Of  these  collenchyma  alone 
is  capable  of  elongating  as  the  growth  in  length  of  a  member 
containing  it  proceeds.  It  is  rather  tough  than  rigid,  and 
offers  a  very  great  resistance  to  any  force  tending  to  tear 
it  in  any  direction.  Sclerenchyma  is  not  extensible,  but 
is  extremely  hard  and  rigid.  In  a  few  delicate  stems  these 
tissues  are  much  more  prominent  than  the  vascular  bundles. 
We  can  notice  three  regions  of  the  stem  or  axis  where  they 
may  appear,  and  in  these  places  they  may  take  the  form 
of  isolated  cells,  or  strands  of  tissue,  or  complete  sheaths 


30  VEGETABLE  PHYSIOLOGY 

going  round  either  the  whole  axis  or  separate  parts  of  it. 
The  first  of  these  regions  is  the  layer  underlying  the 
tegumentary  tissue,  which  the  new  development  sup- 
plements and  strengthens.  Most  moss  plants  show  the 
hypodermal  cells  of  their  axis  thickened,  while  such  a 
development  is  very  common  in  many  petioles  and  leaf- 
blades.  The  new  development  may  occur  in  close  relation- 
ship with  the  vascular  bundles,  which,  in  such  cases,  are 
found  among  large-celled  somewhat  succulent  parenchyma, 


hy 


FIG.  29.— TKANSVEHSE  SECTION  OF  RHIZOME  OF  THE  BBAOKEN  FEKN. 
X  10. 

sc,  bands  of  sclerenchyma ;  hy,  hypodermal  sheath  of  sclerenchyma  ; 
st,  steles ;   ep,  epidermis. 


and  are  not  generally  very  strongly  developed.  The  scler- 
enchyma by  forming  a  separate  sheath  round  each  bundle 
gives  it  a  rigidity  which  it  could  not  derive  from  its  own 
elements,  and  in  addition  prevents  the  whole  stem  from 
being  crushed.  This  arrangement  is  seen  in  the  stems  of 
many  semi-succulent  monocotyledonous  plants,  such  as  those 
of  the  maize  and  the  asparagus  (fig.  31).  The  sclerenchyma 
may  also  occur  freely  in  the  ground  tissue,  at  some  distance 
from  both  tegumentary  and  vascular  structures.  The  bands 
of  it  which  occur  in  the  rhizome  of  the  bracken  fern  are 
good  illustrations  of  this  mode  of  disposition.  The  two 


CALIFORNIA   COLLEGE 
of  PHARMACY 

THE  DIFFEBENTfATION  OF  THE  PLANT-BODY    31 

main  ones  form  an  interrupted  cylinder  (tig.  29),  so  arranged 
as  to  protect  the  delicate  vascular  tissue,  which  is  in  great 
part  placed  either  within  this  cylinder  or  in  some  similar 
relation  to  other  similar  sclerenchymatous  strands.  In  the 
case  of  a  plant  of  humbler  type,  the  common  hair-moss 
(Polytrichum),  a  development  of  somewhat  sclerotised  cells, 
forms  a  central  core  passing  down  the  stem.  In  many  of 
the  flowering  plants  more  complex  distribution  of  scleren- 


FIG.  30. — LEAF  OF  Pinus  (ONE  OF  THE  CONIFEKS) 

ep,  epidermis;    hy,  layer  of  sclerenchyma ;    en,  endodermis ;    v.b.  vascular 
bundle ;  r.d.,  resin  duct. 

chyma  can  be  noticed,  strands  in  the  middle  of  the  cortical 
tissue,  or  in  the  pith  of  the  stem,  being  occasionally  seen. 
Stems  which  are  angular  in  section  are  usually  found  to  have 
their  angles  strengthened  in  a  similar  way. 

The  arrangement  of  this  sclerenchyma  is  generally  such 
as  to  supplement  the  bundles,  and  to  secure  the  greatest 
amount  of  solidity  and  sufficient  flexibility,  with  the  least 
expenditure  of  material. 

Instances  of  various  methods  of  arrangements  of 
strengthening  materials  may  serve  to  illustrate  this  par- 
ticular differentiation  (fig.  32).  In  the  simplest  cases  the 


32  VEGETABLE  PHYSIOLOGY 

sclerenchyma  is  developed  in  connection  with  only  one  of 
the  three  regions  already  alluded  to.  The  stem  of  Equi- 
setum  and  the  leaves  of  Conifers  are  furnished  with  a  layer 
of  thick-walled  cells  immediately  under  the  epidermis 
(fig.  30)  ;  the  vascular  bundles  of  many  Monocotyledons 
are  surrounded  separately  by  a  sheath  of  small  cells  of 


Eia.  31. — VASCULAR  BUNDLE  OF  STEM  OF  MONOCOTYLEDONOUS  PLANT. 
(After  Kny.) 

ph,  phloem ;    x,  xylcm  vessels ;    p  ph,  protophloein.     The  bundle 
is  surrounded  by  a  small-celled  sheath  of  aclerenchyma. 

similar  character  (tig.  31) ;  in  Pennisetum  (tig.  32,  4)  a 
sheath  is  developed  round  the  stem  in  the  form  of  a 
hollow  cylinder  which  lies  between  the  bundles  and  the 
epidermis. 

More  frequent  instances  occur  in  which  two  of  the 
regions  in  question  are  strengthened  simultaneously.  In 
the  stems  of  Scirpus  (fig.  32,  5)  there  is  a  development  of 


THE  DIFFERENTIATION  OF  THE  PLANT-BODY    33 

sclerenchyma  round  the  periphery,  and  strands  occur  also 
in  connection  with  the  bundles.  Sometimes  these  are  con- 
nected by  bands  of  sclerenchyma  lying  between  them.  In 
Fimbristylis  (fig.  32,  7)  there  is  a  ring  of  sclerenchyma  in 
the  cortex  and  patches  around  the  periphery,  which  in  other 
cases  are  joined  like  those  of  the  former  type.  In  the  stems 


lG.  32. — DIAGRAM  SHOWING  THE  CHIEF  DISPOSITIONS  OB1  THE  SKELETAL 
APPARATUS  IN  A  STEM  WITH  FIVE  COLLATERAL  BUNDLES  (IN  TRANSVERSE 
SECTION). 

(The  sclerenchyma  is  black  ;   the  bast  of  the  bundles  is  white ; 
the  wood  is  dotted.) 

1,  Type  without  accessory  sclerenchyma ;  2,  Equiselum ;  3,  Bambusa ; 
4,  Pennisetum  ;  5,  Scirpus  ;  6,  Erianthus  ;  7,  Fimbristylis  ;  9,  Typha  ; 
10,  Juncus  ;  14,  Cladium.  (After  Van  Tieghem.) 


of  Typha  (fig.  32,  9)  a  band  of  sclerenchyma  lies  at  the 
back  of  each  bundle,  and  either  a  ring  or  some  isolated 
strands  may  be  found  in  the  cortex.  The  stem  of 
Jumus  (fig.  32,  10)  shows  these  two  forms  combined 
together. 

Still  more  complicated  cases  show  sclerenchyma  arising 
in  all  three  regions,  sometimes  the  bands  being  all  inde- 
pendent, sometimes  united  in  various  ways.  In  Cladium 
Mariscus  (fig.  32,  14)  those  of  all  the  regions  are  united 

3 


34  VEGETABLE  PHYSIOLOGY 

into  a  continuous  system  which  goes  from  the  teguinentary 
region  towards  the  interior  of  the  stem,  embracing  the 
vascular  bundles  and  attaching  them  to  each  other. 

Similar  differentiation  of  the  supporting  system  is  found 
in  many  leaves,  in  which  it  subserves  the  same  purposes. 
In  many  cases  the  veins  afford  sufficient  protection  against 
tearing  or  rupture  in  consequence  of  violent  winds.  The 
methods  of  their  arrangement  in  many  cases  subserve 
this  purpose  very  completely.  In  other  leaves  of  tough 
leathery  habit  the  delicate  tissue  of  the  mesophyll  is  fre- 
quently protected  from  crushing  by  isolated  thick-walled 
cells  of  curious  shape  which  extend  from  one  epidermis 
to  the  other.  Others  show  bands  of  sclerenchyma  sup- 
plementing the  veins  and  not  infrequently  enclosing  them 
and  reaching  the  epidermis  on  each  side. 

The  supporting  tissue  is  frequently  known  as  the  stereome 
of  the  plant.  It  forms,  as  we  have  seen,  the  most  promi- 
nent part  of  the  endo-skeleton. 

The  cells  of  which  the  masses  of  sclerenchyma  are 
composed  have  been  ascertained  to  possess  almost  as  much 
power  of  withstanding  longitudinal  strain  as  the  finest 
steel,  and  they  are  much  more  ductile  than  either  this 
metal  or  wrought  iron.  Their  arrangement  in  the  different 
ways  described  has  a  very  distinct  relationship  to  the 
character  of  the  strain  they  have  to  resist.  In  such 
structures  as  hollow  stems  where  the  ring  of  bundles  is  but 
slender,  as  they  are  still  required  to  resist  lateral  bending, 
the  additional  supporting  tissue  is  situated  near  the  periphery 
of  the  stem,  and  the  latter  is  often  still  further  strengthened 
by  being  furnished  with  ridges  or  flanges.  An  instance  of  an 
almost  converse  character  is  afforded  by  a  young  root.  In  its 
growth,  while  it  must  possess  sufficient  rigidity  to  enable  it  to 
penetrate  the  soil,  it  must  be  capable  of  frequent  bending  to 
enable  it  to  avoid  obstacles.  This  is  most  advantageously 
provided  for  by  a  solid  central  core  of  tissue,  surrounded  by 
more  succulent  material.  The  transporting  tissue  of  which 
the  centre  is  composed,  which  ultimately  becomes  a  hard 


THE  DIFFEKENTIATION  OF  THE  PLANT-BODY    35 

solid  core  of  wood,  is  comparatively  little  affected  by  the 
flexures  of  the  structure,  and  its  function  is  not  interfered 
with. 

Another  kind  of  differentiation  in  such  a  cell-mass  as 
we  are  dealing  with,  is  the  setting  apart  of  particular 
groups  of  cells  for  various  metabolic 
purposes.  We  have  the  formation  of 
glandular  tissue,  of  the  laticiferous 
systems,  and  so  on.  This  differentia- 
tion may  be  marked  also  by  the  pro- 
duction of  definite  organs  in  the 
protoplasts,  such  as  are  seen  for 
instance  in  the  case  of  the  chloro- 
plasts  of  the  leaves  (fig.  33)  and  FIG  33._^LOKOPLA!m 

Other  green  parts  of  plants.  EMBEDDED  ix  THE  PKO- 

The  habit  of  life  of  a  plant  again        Z"p™CSs™ 
may  influence  its  structure  and  the        OF  A  LEAF. 
degree  of  differentiation  of  its  body 

to  a  very  great  extent.  The  great  group  of  the  Fungi  afford 
us  an  illustration  of  the  degradation  of  structure  which  ac- 
companies a  saprophytic  or  parasitic  habit.  Similar  instances 
of  degradation  are  met  with  among  the  flowering  plants. 

The  needs  of  the  cell-mass  thus  usually  lead  to  the 
differentiation  in  its  substance  of  at  least  four  physiologi- 
cally different  regions — the  tegumentary,  the  conducting, 
the  supporting,  and  the  metabolic.  The  latter  includes  all 
the  parts  in  which  the  protoplasts  are  comparatively  little 
changed,  and  consequently  are  most  concerned  in  carrying 
out  the  vital  processes. 

The  needs  of  the  protoplasts  forming  the  community  of 
the  plant  include,  however,  as  we  have  seen,  something 
more  than  the  arrangements  so  far  described  serve  to  secure 
for  them.  Each  protoplast  must  be  furnished  with  a  certain 
amount  of  air,  or  rather  oxygen.  Almost  all  living  sub- 
stances must  carry  on  during  life  the  process  known  as 
respiration.  The  free-swimming  zoospore  to  which  we 
have  so  often  referred  obtains  a  supply  of  oxygen  from  the 

3* 


36  VEGETABLE  PHYSIOLOGY 

water  in  which  it  lives,  the  gas  being  dissolved  therein. 
Aquatic  plants  also  obtain  their  oxygen  from  this  source, 
but  many  of  them  are  composed  of  a  large  number  of  cells, 
most  of  which  are  situated  at  some  distance  from  the 
exterior.  In  such  plants  large  cavities  or  reservoirs  are 
constructed,  in  which  a  quantity  of  air  is  slowly  accumulated 


FIG.  34. — SECTION  OF  STEM  OF  Potamogeton,  SHOWING  Am  PASSAGES 

IN   THE    COBTEX. 


and  into  which  the  respiratory  products  can  be  discharged. 
From  such  reservoirs  the  oxygen  which  the  cells  require 
is  obtained.  The  composition  of  the  atmosphere  in  these 
chambers  or  lacunae  is  not  accurately  known,  but  it  pro- 
bably differs  somewhat  from  that  of  ordinary  air. 

These  air  passages  or  reservoirs  are  very  conspicuous  in 
the  stalks  of  floating  leaves  such  as  those  of  the  water-lily, 
and  in  the  submerged  stems  of  most  aquatic  plants  (fig.  34). 


THE  DIFFEKENTIATION  OF  THE  PLANT-BODY  37 


A  somewhat  similar  mechanism  is  provided  in  the  case 
of  terrestrial  plants.  At  the  time  of  their  first  formation 
all  the  cells  are  in  close  approximation  to  each  other  at 
all  points  of  their  surface.  This  condition  is,  however, 
only  temporary ;  dur- 
ing the  early  stages  of 
growth  the  cell-walls 
split  apart  at  particu- 
lar places,  usually  the 
angles  of  the  cells.  A 
system  of  intercellular 
spaces  is  thus  formed 
which,  as  growth  pro- 
ceeds, become  con- 
tinuous with  each 
other  and  form  a  sys-  FIG 
tem  extending 
throughout  the  plant. 

They  can  be  detected  in  the  root,  in  the  cortex  of  which 
they  are  conspicuous  (fig.  35) ;  they  may  be  traced  through 


35. — CORTEX  OF  HOOT,  SHOWING  INTER- 
CELLULAR PASSAGES  BETWEEN  THE  CELLS. 


FIG.  36. — SECTION  OF  LEAF  SHOWING  THE  LARGE  INTERCELLULAR  SPACES 
OF  THE  MESOPHYLL. 

all  the  ramifications  of  the  stem,  and  are  seen  to  form  a 
very  prominent  feature  of  the  mesophyll  of  the  leaves  (fig. 
36).  They  communicate  with  the  exterior  in  all  the  green 
parts  of  the  plant,  especially  the  leaves.  In  the  epidermis 
of  all  such  parts  are  small  openings  known  as  stomata 


38 


VEGETABLE  PHYSIOLOGY 


(figs.  37,  88),  which  are  provided  with  two  guard-cells,  by 
the  behaviour  of  which  the  width  of  the  apertures  can  be 


Fro.  37. — PART  OF  LOWER  SURFACE  OF  A  LEAF,  SHOWING  THREE  STOMATA 

IN   DIFFERENT   STAGES   OF   OPENING.       X    300. 


enlarged  or  diminished.     In  those  regions  of  the  axis  where 
corky  layers  cut  off  the  metabolic  tissue  of  the   cortex 


FIG.  38. — SECTION  OF  LOWER   EPIDERMIS   OF 
A  LEAF,  SHOWING  A  STOMA.      x  300. 


from  the  exterior,  certain  other  special  apertures,  the 
knticels,  are  present  (fig.  39).  The  atmosphere  conse- 
quently enters  the  plant  by  these  orifices  and  circulates 


THE  DIFFEBENTIATION  OF  THE  PLANT-BODY  39 

through  the  whole  of  the  intercellular  space  system.  As 
nearly  every  protoplast  abuts  in  part  upon  a  channel  of 
this  system,  its  necessary  aeration  is  secured.  Each  proto- 
plast is  thus  in  a  somewhat  intricate  manner  in  contact  with 
the  external  air,though  really 
situated  perhaps  deep  in  the 
tissues  of  a  plant  of  large 
dimensions. 

Like    the    aquatic  plant, 
the  terrestrial  one  thus  pos- 
sesses  a    reservoir   contain- 
ing   an     atmosphere    which,       Fre<  SO.-SECTTON  OF  A  Lmracm. 
though  its   Composition  may  I,  lenticel ;    per,  cork  layer. 

not  be  exactly  that  of  the 

exterior,  yet  contains  oxygen  for  the  need  of  the  protoplasts 
and  serves  as  the  medium  by  which  all  surplus  carbon  dioxide 
is  removed  from  them. 

This  intercellular  space  system  not  only  subserves  the 
purpose  of  the  gaseous  interchanges  of  respiration,  but 
ministers  in  two  ways  to  the  metabolic  phenomena  carried 
out  by  the  plant.  It  permits  the  access  of  the  atmospheric 
carbon  dioxide  to  structures  in  the  leaves  which  make  it 
available  for  the  construction  of  food  material.  It  further 
is  of  great  importance  in  helping  to  regulate  the  supply  of 
water  to  the  cells.  We  have  seen  that  a  transport  system 
is  differentiated  which  carries  the  water  to  them.  This 
transport  system  does  not,  however,  remove  it  from  them 
subsequently.  The  protoplast  can  only  get  rid  of  water  by 
the  process  of  evaporation,  and  as  it  constantly  needs  a  new 
supply,  it  must  continuously  exhale  watery  vapour  to  make 
room  for  the  incoming  stream.  Such  evaporation  takes 
place  into  the  intercellular  spaces  through  the  delicate  cell- 
walls  which  abut  upon  them.  The  intercellular  reservoir 
contains,  therefore,  an  atmosphere  which  is  charged  almost, 
if  not  quite,  to  saturation  by  aqueous  vapour,  and  under 
ordinary  atmospheric  conditions  this  is  being  continually 
exhaled  as  long  as  an  excess  of  water  is  passing  through 


40  VEGETABLE  PHYSIOLOGY 

the  plant.  The  regulation  of  the  process  of  exhalation 
depends  mainly  upon  the  condition  of  the  guard-cells  of 
the  stomata,  which  can  permit  it  to  go  on  freely  or  can 
check  it  hy  partially  or  entirely  closing  the  apertures, 
according  to  various  internal  and  external  conditions 
(fig.  37). 


41 


CHAPTEK  III 

THE    SKELETON    OF    THE    PLANT 

In  the  last  chapter  we  discussed  the  differentiation  of  the 
body  of  the  plant,  and  examined  the  constitution  of  various 
mechanisms  which  are  associated  with  such  differentia- 
tion. If  we  study  the  arrangements  which  are  peculiar  to 
any  plant,  we  shall  find  that  almost  all  such  differentiation 
as  exists  involves  a  modification  of  the  non-living  part,  and 
particularly  the  walls  of  the  supporting  and  conducting 
tissues,  the  living  protoplasts  having  fundamentally  the 
same  structure  or  composition,  whatever  may  be  the  nature 
of  their  immediate  support.  All  the  various  dispositions 
of  the  non-living  elements  or  structures  are  secondary  in 
importance  to  the  protoplasts. 

We  cannot,  indeed,  lay  too  great  stress  on  the  fact  that 
the  needs  and  conditions  of  the  protoplasts  are  primarily 
the  causes  of  the  differentiation  of  the  non-living  structural 
parts,  and  such  differentiation  is  the  expression  of  the  fact 
that  division  of  labour  has  arisen  among  the  protoplasts  of 
the  community. 

We  have  seen  that  a  protoplast  in  its  simplest  con- 
dition is  capable  of  an  independent  existence  without  any 
form  of  mechanical  support  beyond  that  which  it  derives 
from  the  slight  difference  of  density  between  its  external 
layer  and  its  interior.  In  most  cases,  however,  this  is 
not  sufficient  for  protection  during  its  whole  life,  and  a 
membrane  is  subsequently  formed  around  it.  The  mem- 
brane itself  is  a  secretion  from  the  protoplast,  which  in  fact 


42  VEGETABLE  PHYSIOLOGY 

prepares  its  own  defensive  mechanism.  In  most  cases 
the  protoplast  is  always  clothed  by  a  cell-wall,  the  forma- 
tion of  every  new  cell  being  completed  at  once  by  the 
membrane  which  is  formed  between  the  two  halves  as  soon  as 
the  protoplast  has  divided  into  two.  This  is  particularly 
noticeable  in  cases  where  a  cell-complex  or  community  forms 
the  plant-body.  Each  protoplast  thus  continually  secures 
for  itself  a  chamber  to  dwell  in,  the  walls  of  which  at  first, 
at  any  rate,  are  probably  all  alike,  though  one  is  younger 
than  the  rest.  We  may  consequently  recognise  in  the 
cell- wall  an  exoskeleton  formed  for  itself  by  the  individual 
protoplast,  which  may  or  may  not  undergo  subsequent 
modification. 

In  the  case  of  a  large  plant  consisting  of  innumerable 
protoplasts,  the  cell-walls  of  the  separate  units  are  united 
together  in  various  ways,  and  to  a  different  extent  in  differ- 
ent individuals.  The  resulting  network  constitutes  at  first 
the  skeleton  of  the  whole  plant.  The  modification  of  the 
cell-wall,  which  was  unnecessary  in  the  case  of  a  solitary 
protoplast,  becomes  imperative  as  soon  as  the  needs  of  a 
large  community  are  established,  and  secondary  differentia- 
tions of  such  cell-walls  result,  the  alterations  being  due,  like 
the  original  formation,  to  the  activity  of  the  protoplasts. 
Not  only  are  the  walls  changed  in  substance  and  increased 
in  thickness  after  they  are  formed,  but  the  profoplast  itself 
frequently  alters  the  shape  of  the  cavity  it  has  constructed 
for  itself,  and  consequently  its  own  form,  by  irregularities 
of  subsequent  growth,  which  we  shall  discuss  in  more  detail 
later.  The  skeleton  of  the  plant  comprises  therefore  not 
merely  the  hard  tissues  which  will  survive  maceration  and 
desiccation,  those  coarser  structures  evidently  set  apart  for 
protection  and  support,  but  also  all  the  delicate  cell-walls 
which  form  the  cavities  in  which  the  protoplasts  are  living. 
We  may  indeed  discriminate  between  the  skeleton  of  the 
individual  protoplast  and  that  of  the  large  community  of 
which  it  forms  a  part. 

The  skeleton  of  a  large  plant  such  as  a  tree  increases  in 


THE  SKELETON  OF  THE  PLANT 


43 


complexity  as  its  life  continues.     In  every  plant  growth 
proceeds  continuously  or  intermittently  so  long  as  life  lasts. 


FIG.  40.— DIAGRAM  OF  STEM  OF  DICOTYLEDON:  AT  THREE   AGES. 

A,  Young  condition  showing  the  first  formed  vascular  bundles  a.  B,  A  little  older 
stage,  showing  preparation  for  formation  of  new  conducting  tissue,  c,  Older 
stage.  The  bundles  have  become  larger  and  new  ones  (a)  have  been  intercalated 
between  them. 

Every  year  new  branches  or  twigs  with  their  associated  leaves 
are  constantly  produced.  With  such  continuous  increase 
of  size,  new  conducting  tissue  must  be  formed.  The  skeleton 


44 


VEGETABLE  PHYSIOLOGY 


of  a  young  plant  is  consequently  much  smaller  than  that 
of  an  old  one.  The  difference  between  the  condition 

of  a  stem  at  two  periods  may  be 
seen  by  comparing  fig.  40,  A,  B,  and 
c  ;  A  and  B  show  diagrammatically 
the  arrangement  of  the  supporting 
and  conducting  tissue  at  an  early 
stage  of  its  life,  while  c  indicates 
the  condition  several  months  later. 
During  the  interval  a  large  forma- 
tion of  secondary  vascular  tissue 
has  taken  place,  and  new  bundles 
have  been  intercalated  between  the 
original  ones. 

The  structure  of  a  coenocyte 
shows  a  similar  mode  of  forma- 
tion of  the  skeleton  to  that  of 
a  multicellular  plant-community. 
In  this  case,  however,  the  several 
protoplasts  are  not  furnished  with 
separating  walls.  The  only  skele- 
ton is  the  external  membrane  which 
limits  the  whole  structure,  and 
which  is  formed  by  the  conjoint 
activity  of  them  all.  In  compound 
or  septated  coenocytes  we  have  in 
addition  certain  transverse  walls 
crossing  the  interior  and  giving  a 
greater  degree  of  strength  to  the 
whole  body.  These  separating  walls 
have  a  similar  origin. 

The     primary     cell-wall     which 

clothes  the  unicellular  plant,  and  which  serves  as  the  original 
supporting  membrane  of  the  separate  protoplasts  of  a  com- 
munity or  colony,  is,  when  first  formed,  a  clear,  trans- 
parent, extensible,  and  elastic  membrane.  It  remains  in 
contact  with  the  protoplasm  so  long  as  the  latter  is  living. 


FIG.  41. — EMBRYO  OP  Orobus 

AT  THE  END  OF  A  LONG 
SUSPENSOE,  THE  TWO  SEG- 
MENTS OP  WHICH  HAVE  A 

CffiNocYTic  STRUCTURE. 
(After  Guignard.) 

The  rounded  bodies  in  the  seg- 
ments of  the  coonocytes  are 
the  nuclei  of  the  protoplasts. 


THE  SKELETON  OF  THE  PLANT  45 

Under  certain  conditions  it  is  capable  of  imbibing  con- 
siderable quantities  of  water,  and  in  consequence  swelling 
to  a  greater  or  less  extent.  Under  ordinary  conditions  it  is 
freely  permeable  by  water.  It  is  composed  of  a  substance 
commonly  termed  cellulose,  whose  chemical  composition  is 
represented  by  the  formula  n(C6H1005),  the  value  of  n  not 
having  yet  been  accurately  determined.  This  substance  is 
related  to  such  bodies  as  starch,  sugar,  &c.,  being  a  mem- 
ber of  the  group  of  carbohydrates.  It  is  capable,  under  the 
action  of  hydrating  reagents,  of  being  converted  into  a  form 
of  sugar,  and  under  certain  circumstances  it  can  yield  nutri- 
tive material  for  the  use  of  the  plant.  Cellulose  possesses 
the  peculiar  property  of  becoming  a  deep  blue  in  colour 
when  treated  with  iodine  in  the  presence  of  sulphuric  acid, 
chloride  of  zinc,  'or  other  hydrating  reagent.  It  dissolves 
with  readiness  in  a  solution  of  ammonio-cupric  sulphate 
(Schweizer's  reagent),  but  is  not  soluble  in  dilute  acids  or 
alkalies.  Strong  mineral  acids,  such  as  sulphuric  or  phos- 
phoric, cause  it  to  imbibe  water  and  swell  up,  ultimately 
becoming  gelatinous  and  dissolving.  Certain  soluble  fer- 
ments affect  it  similarly. 

When  the  cell -wall  is  examined  by  polarised  light  it  is 
found  to  be  doubly  refractive. 

It  would  probably  be  more  accurate  to  speak  of  a  group 
of  celluloses,  for  several  varieties  are  known  to  occur  in 
different  parts  of  plants.  We  find  some  kinds  of  it  which 
will  stain  blue  with  iodine  without  previous  hydration. 
Examples  of  this  variety  are  found  in  the  cell-walls  of  the 
bast  of  Lycopodium,  the  endosperm  of  the  Pseony,  the 
cotyledons  of  some  of  the  Leguminosse,  &c.  The  walls  of 
the  hyphaB  of  the  fungi  are  peculiar,  in  that  they  will  not 
give  the  blue  colour  with  iodine  even  after  treatment  with 
hydrating  reagents.  Kecent  observations  suggest  that  this 
variety  ot  cell-wall  approaches  in  composition  the  chitin  of 
the  animal  kingdom. 

The  celluloses  which  have  been  so  far  examined  have 
been  divided  into  three  categories,  according  to  the  ease 


46  VEGETABLE  PHYSIOLOGY 

with  which  they  can  be  made  to  undergo  hydrolysis,  and  to 
yield  some  variety  of  sugar  by  such  treatment.  The  cellu- 
loses of  cotton  fibres  are  perhaps  the  most  resistent  of  all, 
and  may  be  taken  as  representatives  of  the  most  refractory 
group.  The  cellulose  found  in  the  main  mass  of  the  funda- 
mental tissue  of  the  flowering  plants  is  less  resistent,  giving 
very  easily  the  reactions  which  have  been  just  described. 
A  third  variety  is  hydrolysable  with  still  greater  readiness. 
It  is  to  a  certain  extent  soluble  in  alkalies,  and  is  easily 
decomposed  by  acids  with  formation  of  other  carbohydrates 
of  low  molecular  weight.  Such  cellulose  is  represented  in 
the  cell-walls  of  most  seeds. 

It  is  probable  that  cellulose  is  chemically  combined 
with  a  certain  amount  of  water,  and  that  the  degree  of  such 
hydration  differs  in  the  different  varieties  described. 

Though,  as    already   stated,  the  cell-wall   is  commonly 

said  to  be  composed  of  cellulose,  the  latter  material  is  always 

associated  with  other  constituents.     Among  the  latter  we 

find  various  members  of  another  group  known  as  pectoses, 

which   differ  in  many  ways   from   cellulose.     This   group 

includes  two  series  of  bodies  which  vary  among  themselves 

as  to  the  degree  of  their  solubility  in  water.     One  of  these 

series  comprises  bodies  of  a  neutral  reaction,  while  those 

of  the  other  are  feeble  acids.     In  each  series  there  are 

probably  several  members,  which  show  among  them  every 

stage  of  physical  condition  between  absolute  insolubility 

and  complete  solubility  in  water,  the  intermediate  bodies 

exhibiting  gelatinous  stages,  characterised  by  the  power  of 

absorbing  water  in  a  greater  or  less  degree. 

Of  the  neutral  series  the  two  extremes  are  known  as 
pectose  and  pectine.  The  former  is  insoluble  in  water  j  and 
is  closely  associated  with  cellulose  in  the  substance  of  most 
cell-walls  ;  the  latter  is  soluble  in  water  and  forms  a  jelly 
with  more  or  less  facility.  Pectose  has  not  yet  been  obtained 
pure,  in  consequence  of  its  close  association  with  cellulose 
and  the  readiness  with  which  it  undergoes  change  in  the 
process  of  extracting  it.  The  reagents  which  separate  it 


THE  SKELETON  OF  THE  PLANT  47 

from  cellulose  convert  it  into  pectine,  or  into  pectio  acid, 
the  former  being  soluble  in  water,  the  latter  in  alkalies. 
The  cell -wall  can  be  shown  to  contain  the  two  constituents 
by  the  action  of  Schweizer's  reagent,  which,  when  used 
with  proper  precautions,  dissolves  out  the  cellulose  and 
leaves  the  framework  of  the  cell  apparently  unaltered  ; 
it  consists  then,  however,  not  of  pure  pectose,  but  of  a 
compound  of  pectic  acid  with  some  of  the  copper  of  the 
reagent. 

Pectine  swells  up  and  dissolves  in  water,  forming  a 
viscous  liquid  which  soon  becomes  a  jelly.  It  exists  in 
considerable  quantity  in  many  ripe  fruits  and  in  some 
mucilages.  It  gives  no  precipitate  with  the  neutral  acetate 
of  lead,  but  is  thrown  down  by  the  basic  acetate  in  the  form 
of  white  liocculi.  If  it  is  boiled  for  some  hours  in  water,  it 
is  converted  into  parapectine,  which  is  precipitated  by 
neutral  lead  acetate.  Further  boiling  with  dilute  acids 
converts  it  into  metapectine,  which  can  be  precipitated  by 
barium  chloride. 

The  acid  series  shows  peculiarities  similar  to  those  of  the 
neutral  one.  Its  most  insoluble  member  is  pectic  acid,  which 
will  not  dissolve  in  water,  alcohol,  or  acids  ,  it  forms  soluble 
pectates  with  alkalies,  and  insoluble  ones  with  the  metals 
of  the  alkaline  earths,  of  which  calcic  pectate  is  the  most 
widely  distributed.  It  dissolves  in  solutions  of  alkaline 
salts,  such  as  the  carbonates  of  sodium  and  potassium, 
alkaline  phosphates  and  most  organic  ammoniacal  salts, 
forming  with  them  double  salts  which  gelatinise  more  or 
less  freely  with  water.  Its  solution  in  alkaline  carbonates 
is  mucilaginous,  but  when  ammonic  oxalate  is  the  solvent 
it  is  perfectly  limpid. 

The  member  at  the  other  end  of  the  series  is  metapectic 
acid,  a  body  with  an  acid  reaction,  freely  soluble  in  water 
and  forming  soluble  salts  with  all  bases,  especially  those  of 
calcium  and  barium,  which  precipitate  pectic  acid.  Meta- 
pectates  are  coloured  yellow  when  they  are  warmed  with  an 
excess  of  alkali.  This  body  and  its  compounds  are  probably 


48  VEGETABLE  PHYSIOLOGY 

very  prominent  in  the  gums  ;  when  acted  on  by  dilute 
sulphuric  acid  they  split  up,  one  of  their  products  being 
a  crystallisable  dextro-rotatory  sugar  which  is  apparently 
arabinose.  Metapectic  acid  does  not  form  a  jelly,  its  solu- 
tions always  being  limpid. 

The  two  series  of  pectic  bodies  are  closely  related  to 
each  other,  for  by  the  action  of  heat,  acids,  and  alkalies 
the  various  members  of  both  can  be  prepared  from  pectose. 
The  final  product  of  the  action  of  the  reagents  is  the  freely 
soluble  metapectic  acid. 

The  cellulosic  and  pectic  constituents  of  the  cell-wall 
show  considerable  differences  of  behaviour.  The  former 
are  soluble,  the  latter  insoluble,  in  Schweizer's  reagent ; 
when  oxidised  with  nitric  acid  the  former  yield  oxalic,  the 
latter  mucic  acid.  The  celluloses  when  partially  hydrated 
stain  blue  with  iodine  ;  the  pectic  bodies  give  no  coloration 
with  this  reagent.  They  behave  differently  also  to  staining 
reagents  and  to  dilute  acids  and  alkalies. 

The  celluloses,  as  we  have  seen,  are  members  of  the 
group  of  carbohydrates.  Various  writers  are  not  agreed 
as  to  the  relation  of  the  pectic  bodies  to  this  group,  some 
holding  that  their  reactions  separate  them  from  it  entirely, 
while  others  contend  that  they  are  closely  connected  with 
it,  if  they  do  not  actually  belong  to  it.  It  has  been  suggested 
that  they  are  carbohydrates  chemically  combined  with  acids. 
Like  the  celluloses,  they  yield  some  form  of  sugar  when 
hydrolysed  with  dilute  mineral  acids. 

All  unchanged  cell- walls  contain  a  varying  quantity  of 
water,  and  various  views  have  been  advanced  as  to  the  way 
in  which  the  latter  is  held  by  the  other  constituents.  It  is 
probably  not  in  a  state  of  chemical  union,  as  the  quantity 
present  can  be  easily  increased  or  diminished. 

Naegeli  suggested  that  the  wall  contained  particles  of 
solid  matter  or  micellce,  of  crystalline  form,  the  long  axis 
of  the  crystals  being  arranged  at  right  angles  to  the  surface 
of  the  wall.  He  supposed  each  micella  to  be  surrounded 
by  a  thin  film  of  water.  Every  cell-wall  is  thus  under  some 


THE  SKELETON  OF  THE  PLANT  49 

considerable  internal  strain,  the  micellae  attracting  each 
other  and  tending  to  squeeze  out  the  water.  The  latter,  on 
the  other  hand,  tends  to  separate  the  micellae. 

According  to  Strasburger,  the  molecules  of  the  solid 
matter  are  held  together  by  chemical  affinity,  and  there  is 
no  definite  aggregation  of  them  into  micellae.  He  pictures, 
therefore,  a  linkage  of  the  atoms  into  a  molecular  network, 
the  meshes  of  which  are  occupied  by  water.  On  either 
hypothesis  the  quantity  of  water  is  capable  of  considerable 
increase  or  diminution,  and  the  wall  can  be  made  to  swell 
up  by  causing  it  to  imbibe  more  fluid.  This  can  be  brought 
about  by  exposing  it  to  the  action 
of  strong  mineral  acids,  such  as 
sulphuric  acid.  The  water  is  held, 
however,  by  the  solid  particles 
with  very  great  tenacity. 

A  different  view  of  the  composi- 
tion of  the  cell-wall  was  advanced 
some  years  ago  by  Wiesner.  He 
held  that  the  substance  of  the 
membrane  as  it  is  first  formed  FlG>  42.-THicKENED  CELLS  OF 

Consists  Of  rOWS  Of  granular  bodies        WoOD>     SHOWING     STRATIFICA- 
,  .  .  7  TION.     (After  Sachs.) 

which   he   termed    dermatosomes ; 

these  are  connected  together  by  protoplasm  which  surrounds 
them.  He  based  his  view  on  the  phenomena  which  accom- 
pany the  disintegration  of  the  wall  by  the  action  of  strong 
alkalies.  On  this  hypothesis  the  cell- wall  is  living  while 
young  and  growing.  The  protoplasm  exists  in  it  between 
particles  of  solid  matter,  and  it  holds  the  water  in  its 
substance. 

The  thickening  which  always  supervenes  to  a  greater 
or  less  extent  upon  the  first  formation  of  the  cell- wall  is 
brought  about  by  the  protoplasm  in  a  way  similar  to  the 
method  of  its  original  construction.  Layers  composed  like 
the  original  one  are  continually  secreted  by  the  protoplast, 
and  are  deposited  upon  its  exterior  in  apposition  with  the 
wall  already  there.  Hence  walls  which  have  a  perceptible 

4 


50 


VEGETABLE  PHYSIOLOGY 


thickness  show  a  certain  stratification,  which  is  most  easily 
seen  in  transverse  sections  (fig.  42).  When  several  such 
layers  can  be  distinguished  it  has  been  found  that  pectic 
bodies  are  prominent  in  the  layers  furthest  from  the  proto- 
plasm, and  cellulose  in  those  nearest  the  interior  of  the  cell. 
The  action  of  the  protoplast  is  frequently  irregular,  so  that 
the  thickening  layers  are  often  seen  as  bands  of  various 
form,  giving  the  surface  of  the  membrane  particular  patterns, 
thin  and  thick  places  alternating  in  various  ways  (fig.  43). 


FIG.  43. — LONGITUDINAL  SECTION  OF  VASCULAR  BUNDLE  OF  SUNFLOWER 
STEM.     (After  Prantl.) 

a,  Vessels  of  the  wood  thickened  in  various  ways. 

These  are  seen  most  conspicuously  in  the  walls  of  the  vessels 
of  the  wood. 

In  some  cases  the  thickening  is  caused  or  materially 
aided  by  the  intercalation  of  fresh  molecules  of  cellulose 
into  the  substance  of  the  existing  wall.  This  process  is 
known  as  intussusception.  It  appears  to  be  not  so  general 
as  was  formerly  supposed. 

In  cell- walls  which  have  undergone  considerable  thickening 
the  membrane  shows  a  marked  differentiation.  The  centre 
of  the  wall  is  found  to  possess  a  chemical  composition 


THE  SKELETON  OF  THE  PLANT      51 

very  unlike  that  of  the  thickening  layers.  It  marks 
off  the  limits  of  the  cells,  occupying  the  position  of  the 
original  thin  membrane,  and  looking  as  if  it  were  the  basis 
on  which  the  thickening  layers  have 
been  deposited.  When  a  piece  of 
tissue  is  warmed  gently  with  a 
mixture  of  potassic  chlorate  and 
strong  nitric  acid,  this  layer  dis- 
solves and  the  cells  become  separated 
from  each  other.  It  has  by  certain 
writers  been  termed  the  intercellular 
substance  and  by  others  the  middle 
lamella  (fig.  44).  Though  it  is  most 
easily  seen  in  thickened  cells,  it  is  FIG.  44.— THICKENED  WOOD- 

.,  CELLS,     SHOWING     MIDDLE 

probably  not  connned  to  them,  but       LAMELLA.   (After  Sachs.) 
exists   in  all  cell  -  membranes,  even 

when  they  are  very  young.  Treatment  with  the  reagent 
mentioned  will  disintegrate  the  tissue  of  even  the  growing 
points  of  stems  and  roots,  and  will  cause  their  cells  to 
become  isolated.  A  thin  layer  of  this  nature  therefore 
probably  exists  even  in  the  primary  cell-wall.  It  is  added 
to  materially,  however,  during  the  growth  in  thickness  of 
the  walls,  and  in  many  cases  it  can  be  seen  easily  under 
a  comparatively  low  magnification. 

This  middle  lamella  is  composed  of  a  material  which  is 
very  unlike  that  of  the  rest  of  the  cell-wall.  Besides  dis- 
solving readily  under  the  action  of  potassic  chlorate  and  nitric 
acid,  which  do  not  affect  the  inner  layers  of  the  membrane,  it 
resists  completely  the  action  of  sulphuric  and  other  mineral 
acids,  which  cause  the  inner  layers  to  swell  and  ultimately 
to  dissolve.  Kecent  investigations  have  led  to  the  view 
that  it  is  composed  of  a  calcium  salt  of  pectic  acid. 

Whether  the  primitive  cell-wall  is  homogeneous  or  not  is 
uncertain.  If  it  is,  it  must  be  regarded  as  being  formed  of  an 
intimate  mixture  or  perhaps  of  a  compound  of  cellulose  and 
pectose  constituents.  At  a  very  early  period  in  its  develop- 
ment the  middle  lamella  becomes  differentiated,  owing 

4* 


52  VEGETABLE  PHYSIOLOGY 

possibly  to  the  conversion  of  the  pectose  into  pectic  acid  and 
the  interaction  of  the  latter  with  some  salt  of  calcium  derived 
from  the  cell-sap  which  infiltrates  the  wall.  The  calcium 
pectate  becomes  deposited  in  this  way  halfway  between 
the  contiguous  cells  which  are  separated  by  the  particular 
membrane  in  which  the  change  is  taking  place. 

If  the  cell-wall  is  not  at  first  homogeneous,  we  must 
suppose  that  the  original  thin  membrane  is  composed  of 
three  layers,  a  central  one  of  calcium  pectate,  on  each  face 
of  which  is  a  layer  composed  of  the  mixture  or  compound 
of  cellulose  and  pectose.  We  never  find,  even  at  the  moment 
of  cell  division,  that  the  membrane  is  formed  of  calcium 
pectate  only. 

It  is  possible  to  explain  the  growth  in  thickness  of  the 
middle  lamella  on  either  hypothesis.  It  is  clear  that  the 
wall  is  the  seat  of  a  considerable  chemical  change  which 
affects  its  whole  substance,  though  the  degree,  and  possibly 
the  character,  of  the  change  may  vary  in  the  different 
layers  of  which  the  wall  is  built  up. 

Not  infrequently  it  is  noticeable  that  the  intercellular 
spaces  contain  small  concretions  of  various  form,  which 
consist  of  the  same  substance  as  the  middle  lamella.  This 
is  scarcely  to  be  wondered  at,  as,  when  the  intercellular 
spaces  are  formed  by  the  splitting  of  the  cell-wall,  the 
region  of  the  middle  lamella,  which  is  the  central  part  of 
the  membrane,  must  abut  upon  the  space  formed  in  the 
rupture.  The  calcium  pectate  which  is  formed  or  deposited 
in  the  central  region,  and  which  causes  the  thickening  of 
the  middle  lamella,  may  well  exude  to  a  certain  extent  into 
the  intercellular  space  that  has  been  formed. 

In  such  parts  of  the  framework  of  a  well-differentiated 
plant-body  as  need  considerable  rigidity,  a  conversion  of 
cellulose  into  lignin  takes  place.  This  material  is  found 
conspicuously  in  the  walls  of  wood-cells  and  sclerenchyma. 
It  is  formed  in  the  substance  of  the  cell -wall,  and  in  parti- 
ally lignified  membranes  the  lignin  can  be  dissolved  out 
by  appropriate  reagents,  leaving  a  cellulose  basis.  In  its 


THE  SKELETON  OF  THE  PLANT      53 

chemical  characters  lignin  differs  remarkably  from  cellulose. 
It  does  not  stain  blue  with  iodine  and  sulphuric  acid,  but 
can  be  recognised  by  its  property  of  becoming  red  when 
treated  with  phloroglucin  and  a  mineral  acid,  or  yellow 
with  anilin  chloride  under  the  same  conditions.  Its  physical 
properties  are  also  different,  and  bear  a  definite  relation  to 
the  function  of  the  tissue  as  "a  medium  for  the  transport  of 
water.  It  has  little  extensibility,  nor  can  it  imbibe  water 
and  swell  as  can  unaltered  cell-wall ;  on  the  other  hand,  it 
allows  water  to  pass  through  it  with  great  rapidity  and 
ease. 

Lignin  is  probably  not  a  definite  chemical  compound, 
but  a  mixture  of  substances  formed  successively  from  the 
cellulose. 

Walls  containing  it  subserve  a  double  purpose.  Its 
physical  properties  render  it  particularly  adapted  to  serve 
as  the  material  of  which  the  tissues  conducting  the  stream 
of  water  are  composed.  Its  deficiency  in  flexibility  or 
extensibility  makes  it  suitable  for  the  securing  of  rigidity 
in  tissues  or  structures  needing  considerable  power  of  re- 
sistance to  winds  or  storms.  It  is  thus  a  valuable  material 
in  the  construction  of  sclerenchyma. 

The  protective  tissues  show  a  different  modification  of 
the  original  structure.  In  the  simplest  cases  we  have  seen 
that  the  degree  of  protection  secured  is  slight,  and  evidently 
only  transitory.  The  epidermis  is,  in  these  cases,  the  seat 
of  the  changes  which  may  be  observed.  The  cells  show 
their  walls  sometimes  very  materially  thickened  on  the 
exposed  sides  (fig.  45),  though  the  thickness  varies  in 
different  cases.  Layer  after  layer  of  substance  is  deposited 
upon  the  original  wall  in  these  regions,  the  other  parts  of 
it  remaining  thin.  The  thickness  itself  secures  a  cer- 
tain amount  of  protection  against  cold,  but  to  prevent 
absorption  or  dissipation  of  water  or  of  gases  by  these 
membranes,  a  chemical  change  also  is  brought  about. 
The  outer  layers  of  the  wall  undergo  a  process  known  as 
cuticularisation,  which  generally  extends  about  halfway 


54  VEGETABLE  PHYSIOLOGY 

through  its  thickness.  This  change  in  the  outer  walls 
of  numbers  of  contiguous  cells  renders  it  possible  to 
strip  off  from  such  a  tissue  a  piece  of  apparently  structure- 
less membrane,  which  is  technically  called  the  cuticle,  and 
which  consists  of  nothing  more  than  these  altered  layers  of 
the  outermost  walls  of  the  contiguous  cells.  The  alteration 
of  the  chemical  character  of  this  membrane  in  forming  the 
cuticle  of  the  epidermis  is  due  to  the  transformation  of  its 
cellulose  or  pectose  constituents  into  a  substance  known  as 
cutin.  Its  properties  are  very  different  from  those  of  the 


FIG.  45. — SECTION  THROUGH  EPIDERMIS  OF  LEAF, 
SHOWING  THE  OUTER  WALLS  MATERIALLY 
THICKENED  AND  CUTICULARISED. 

a,  epidermis  ;  &,  cells  of  mesophyll. 

original  cell-wall ;  it  is  but  slightly  permeable  by  water, 
and  it  is  not  easy  for  gases  to  pass  into  or  through  it. 
This  difference  of  physical  property  is  accompanied  by 
characteristic  reactions  ;  it  stains  yellow  instead  of  blue 
when  treated  with  iodine  and  sulphuric  acid,  and  becomes 
brown  under  the  action  of  strong  alkalies,  such  as  caustic 
potash. 

More  efficient  and  prolonged  protection  is  afforded  by 
the  formation  of  sheaths  of  cork,  certain  layers  being 
differentiated  as  meristem  tissue,  or  actively  dividing  cells, 
for  the  continued  production  of  this  material.  The  walls 
of  true  cork  cells  are  thin,  but  the  presence  of  cutin  is  a 
conspicuous  feature  in  them.  They  are  very  regular  in 
form,  and  are  closely  arranged  together  without  any  inter- 
cellular spaces  (fig.  46).  Coming  as  they  do  between  the 


THE  SKELETON  OF  THE  PLANT 


55 


exterior  and  the  metabolic  tissue  of  the  cortex  of  stems, 
thus  cutting  off  the  intercellular  space  system  of  the  latter 
from  access  to  the  air,  they  are  usually  penetrated  by  special 
structures  known  as  lenticels.  These  are  made  up  of  corky 
cells  very  loosely  arranged,  and  consequently  set  up  the 
communication  needed  (fig.  47).  During  the  winter  a 
layer  of  cork  is  formed  below  the  lenticel. 

In  the  corky  cell-wall  the  cutin  is  frequently  associated 
with  a  certain  amount  of  lignin. 

The  thin  corky  walls  possess  almost  exactly  the  same 
physical  properties  as  the  thickened  cuticle  of  the  epidermis, 
a  fact  which  affords  evidence  that  the  primary  function  of 
both  is  the  same. 


FIG.  46. — OUTER  PORTION  OF  CORTEX 

OF  YOUNG  TWIG  OF  LIME. 
per,  cork  layer ;   ph,  meristem  layer. 


FIG.  47. — SECTION  OF  A  LENTICEL. 
/,  lenticel ;  per,  cork  layer. 


Like  the  substance  of  the  middle  lamella,  both  lignin 
and  cutin  are  soluble  in  warm  nitric  acid  containing  potassic 
chlorate. 

In  some  cases  the  cell-walls  of  the  epidermal  protoplasts 
are  impregnated  with  various  matters  that  do  not  proceed 
from  their  own  disintegration.  Among  these  are  various 
fatty  bodies,  while  wax  is  sometimes  very  conspicuous. 
The  Uoom  of  such  fruits  as  the  grape  and  the  plum  is 
composed  of  very  fine  waxy  particles  ;  the  impregnation 
in  their  case  having  been  so  great  that  certain  particles 
have  passed  beyond  the  walls  and  formed  a  layer  on  the 
outer  surface.  The  leaves  of  the  wax-palm  show  an  even 
denser  deposit. 

Mineral  matters  are  also  of  frequent  occurrence  in  the 


56 


VEGETABLE  PHYSIOLOGY 


cell-\vall.  The  chief  of  these  are  salts  of  calcium,  usually 
the  oxalate,  but  often  the  carbonate.  Some  cell-walls  show 
a  copious  deposit  of  regular  crystals  of  one  of  these — such 
are  the  cells  of  the  bulb  scales  of  the  onion,  the  fibres  of 
the  bast  of  Ephedra,  among  others  (fig.  48).  In  many  plants 
copious  deposits  of  silica  are  formed  in  the  cell-wall, 
especially  in  the  epidermal  cells  of  the  Equisetacece,  and  in 
those  of  the  cereal  grasses.  The  value  of  this  deposit  to 
the  plant  is  not  very  evident ;  it  appears  at  first  sight  to  be 
an  adaptation  enabling  the  plant  to  remain  upright,  but  it 


FIG.  48. — CRYSTALS  or  CALCIUM 
OXALATE  IN  WALL  OF  CELL 
OF  THB  BAST  OF  Ephedra. 


FIG.  49. — SECTION  OF  PORTION 
OF  LEAF  OF  Ficus,  SHOW- 
ING CYSTOLITH  (cys)  IN  LARGE 
CELL  OF  THE  THREE-LAYERED 
EPIDERMIS  (ep).  (pa)  PALI- 
SADE LAYER. 


is  found  that  its  absence  does  not  render  the  grasses  more 
liable  to  fall. 

Some  cells  of  the  epidermis  of  certain  plants,  especially 
among  the  Nettle  family,  contain  curious  ingrowths  of 
cellulose,  in  which  there  is  a  very  large  deposition  of 
calcium  carbonate.  They  are  known  as  cystoliihs  (fig.  49). 

The  cell-walls  of  certain  regions  of  particular  plants 
are  transformed  into  mucilage.  This  material  is  especially 
prominent  in  the  large  brown  seaweeds,  particularly  the 
Fucacece,  where  it  forms  the  bulk  of  the  internal  tissue.  It 
occurs  also  in  certain  layers  of  the  seed-coats  of  such  seeds  as 
linseed,  and  in  certain  regions  in  the  sporocarps  of  Marsilea. 


THE  SKELETON  OF  THE  PLANT      57 

It  is  of  assistance  in  the  dissemination  of  the  spores  of 
this  plant,  and  possibly  has  a  similar  value  in  the  cases 
of  such  seeds  as  contain  it.  It  differs  from  cellulose  by 
imbibing  water  greedily,  and  swelling  up  considerably. 
It  gives  a  blue  colour  with  iodine  and  sulphuric  acid  as 
cellulose  does,  differing  from  the  latter  chiefly  in  the  ease 
with  which  the  imbibition  of  water  is  brought  about.  It 
is  not  clear  at  present  whether  mucilage  is  derived  from 
cellulose  only,  or  whether  the  pectoses  take  part  in  its  com- 
position, though  the  latter  is  probable.  The  gums  are  closely 
related  to  mucilage,  and  seem  to  represent  a  further  dis- 
integration of  the  cell-wall  in  that  direction.  Many  of  the 
gums  yield  derivatives  much  like  those  of  pure  pectic 
bodies,  which  suggests  that  their  affinities  are  rather  with 
the  latter.  In  all  probability,  however,  they  are  all  mix- 
tures of  the  two  classes  of  constituents. 

We  see  thus  that  in  the  construction  of  the  skeleton  of 
a  complex  plant,  while  its  basis  is  the  cell-membranes  of 
the  several  protoplasts,  which  at  first  form  an  almost 
homogeneous  tissue,  not  only  does  differentiation  take 
place  in  the  direction  indicated  in  the  last  chapter,  but 
this  differentiation  is  accompanied  by  changes  in  chemical, 
physical,  and  mechanical  properties,  which  fit  the  definite 
adult  tissues  to  perform  the  functions  which  fall  to  them. 
Temporary  structures  generally  possess  a  different  chemical 
composition  from  permanent  ones.  The  transitory  cuticle 
gives  place  to  the  more  permanent  cork,  and  this  becomes 
strengthened  by  the  introduction  of  sclerenchymatous 
elements  as  the  cork  formation  becomes  continuously  more 
deep-seated.  The  strengthening  tissue  varies  similarly; 
the  walls  of  collenchyma,  though  thickened  in  a  particular 
way,  are  not  chemically  changed  in  the  same  manner  as  those 
of  sclerenchyma  or  woody  tissue,  for  their  cellulose  under- 
goes no  conversion  into,  or  impregnation  with,  lignin. 
The  fibres  of  the  bast  differ  from  those  of  the  wood  in  the 
same  particulars. 


58 


GHAPTEE  IV 

THE  KELATION  OF  WATER  TO  THE  PROTOPLASM  OF  THE   CELL 

When  we  regard  the  arrangement  of  protoplasm  in  the 
cells  of  the  plant,  or  observe  the  environment  of  the  free- 
swimming  protoplast,  we  notice  especially  its  very  close 
relation  to  water.  The  naked  zoospore  is  naturally 
saturated  with  the  latter,  being  in  the  fullest  contact  with 
it.  Unicellular  plants  which  are  not  actually  immersed  in 
it  are  generally  to  be  found  in  more  or  less  moist  situa- 
tions, where  they  continually  obtain  supplies  from  dew  or 
rain.  Indeed,  in  times  of  drought  when  moisture  is  not 
supplied  to  them  they  are  seriously  injured.  The  young 
cell  which  is  clothed  with  a  cell-membrane  speedily  shows 
a  tendency  to  accumulate  water  in  its  interior  ;  gradually 
drops  appear,  which  lead  ultimately  to  the  formation  of  a 
vacuole,  \vhich  is  always  full  of  liquid.  In  a  plant  which 
consists  of  a  complex  of  cells,  such  a  vacuole  is  found  in 
every  adult  cell  so  long  as  it  is  living.  The  healthy  proto- 
plasm is  thus  always  in  contact  with  water.  Indeed,  the 
molecular  constitution  of  protoplasm,  as  far  as  we  know  it, 
lends  itself  to  this  relation,  for  the  apparently  structureless 
substance  is  always  saturated  with  it.  It  is  only  while  in 
such  a  condition  that  active  life  can  exist ;  with  very  rare 
exceptions,  if  a  cell  is  once  completely  dried,  even  at  a  low 
temperature,  its  life  is  gone,  and  restoration  of  water  fails 
to  enable  it  to  recover. 

The  constancy  of  the  occurrence  of  the  vacuole  in  the 
cells  of  the  vegetable  organism  is  itself  very  strong  evidence 


KELATION  OF  WATER  TO  THE  PROTOPLASM     59 

that  such  cells  are  dependent  upon  water  for  the  main- 
tenance of  life.  The  cell-wall,  though  usually  permeable, 
yet  presents  a  certain  obstacle  to  the  absorption  of  water, 
and  so  even  those  cells  which  are  living  in  streams  or 
ponds  usually  possess  a  vacuole.  Cells  without  a  mem- 
brane, such  as  the  zoospores  already  many  times  men- 
tioned, can  more  readily  absorb  water  from  without,  and 
hence  they  are  not  vacuolated  to  the  same  extent  as  are 
those  which  possess  a  cell-wall ;  indeed,  many  of  them 
have  no  vacuole.  This  cavity  when  present  being  always 
filled  with  liquid,  the  protoplasm  of  the  cell  has  ready 
access  to  water,  as  much  so,  indeed,  as  the  protoplast  which 
possesses  no  cell-wall.  The  vacuole  contains  a  store  which 
is  always  available. 

The  quantity  of  water  which  a  vacuole  can  contain  is 
very  small,  and  as  the  needs  of  the  protoplasm  are  some- 
what extensive,  a  need  arises  for  the  continual  renewing  of 
its  supply.  This  is  evident  when  we  consider  that  the 
protoplasm  draws  its  nutriment  eventually  from  the  water, 
and  that  it  must  return  to  it  such  waste  products  as  it 
gives  off.  Its  oxygen  must  be  drawn  from  the  same 
source,  for  this  gas  can  only  pass  into  the  interior  of  a  cell 
in  solution  in  the  liquid  which  enters  it.  In  cells  which  are 
deep-seated  the  need  of  oxygen  can  only  be  supplied  by  a 
slow  passage  from  cell  to  cell  of  the  gas  which  has  been 
dissolved  by  those  abutting  upon  a  free  surface,  or  is  already 
in  solution  in  the  water  absorbed  by  the  roots.  Similar 
considerations  apply  to  the  elimination  of  the  carbon 
dioxide  which  accompanies  the  respiratory  processes. 

The  life  of  a  plant  is  consequently  very  intimately  con- 
nected with  the  renewal  of  the  water  which  the  cells  contain. 
Fresh  liquid  must  be  taken  in,  and  that  which  is  already 
there  must  be  to  a  certain  extent  removed  ;  the  plant 
demands  in  fact  a  kind  of  circulation  of  water,  and  this 
becomes  the  more  imperative  as  the  mass  of  the  plant 
increases,  with  the  possible  exception,  however,  of  those 
massive  plants  whose  habitat  is  marine. 


60 


VEGETABLE  PHYSIOLOGY 


In  examining  the  way  in  which  this  circulation  is  set 
up  and  maintained,  it  is  first  necessary  to  inquire  into  the 
nature  of  the  way  in  which  water  makes  its  entry  into 
a  cell.  This  is  based  upon  a  physical  process  which  is 
known  as  osmosis. 

When  two  fluids  of  different  concentration,  such  as  water 
and  sugar  solution,  are  separated  from  each  other  by  a  homo- 
geneous permeable  membrane,  they  will  tend  to  pass  through 
the  latter  in  both  directions  till  there  is  a  mixture  of  the  two  of 
equal  density  on  each  side  of  it.  We  shall  thus  have  a  stream 
of  water  passing  through  the  membrane  to  the  syrup,  and  a 
stream  of  syrup  similarly  passing  to  the  water.  The  rate 
of  flow  of  the  two  streams  will  not  be  the  same,  however, 
and  the  first  result  will  be  a  considerable  increase  of  the 
volume  of  the  liquid  upon  the  side  of 
the  membrane  in  contact  with  the  syrup, 
owing  to  the  greater  amount  of  water 
that  will  have  passed  through. 

A  convenient  form  of  apparatus  to 
exhibit  this  process  of  osmosis  is  repre- 
sented in  fig.  50.  It  consists  of  a 
bladder  fastened  to  the  end  of  a  narrow 
tube  which  is  immersed,  as  shown,  in  a 
vessel  of  water.  The  bladder  and  part 
of  the  tube  are  filled  with  syrup,  and 
the  height  at  which  the  latter  stands  in 
the  tube  is  noted.  After  some  time  the 
contents  of  the  tube  will  be  increased  in 
consequence  of  the  entry  of  water  being 
greater  than  the  escape  of  syrup,  and 
the  liquid  will  stand  at  a  higher  level 
in  the  tube.  If  the  positions  of  the  water  and  the  syrup 
had  been  reversed,  the  liquid  would  have  fallen  in  the  tube, 
showing  that  the  greater  osmotic  stream  was  in  the  opposite 
direction. 

The  relative  difference  in  the  rate  of  the  two  streams 
will  vary  with  the  concentration  of  the  syrup. 


FIG.  50.— APPARATUS 

TO  SHOW   THE   PRO- 
CESS OF  OSMOSIS. 


EELATION  OF  WATEK  TO  THE  PBOTOPLASM     61 

Other  substances  than  sugar  have  a  similar  power  of 
setting  up  osmotic  currents,  which,  indeed,  is  especially  pro- 
minent in  those  which  are  crystalloid  in  character,  though  it 
is  not  confined  to  them.  Solutions  containing  different  sub- 
stances in  equal  percentages  do  not,  however,  possess  equal 
osmotic  powers  ;  each  one  has  its  own  special  ability  and 
is  said  to  exert  its  particular  osmotic  pressure.  With  any  par- 
ticular substance,  however,  the  osmotic  pressure  is  approxi- 
mately proportional  to  the  concentration  of  the  solution. 

The  simple  process  of  osmosis  is  capable  of  substantial 
modification  according  to  the  character  or  composition  of 
the  membrane.  While  a  diaphragm  of  bladder  or  of  veget- 
able parchment  will  allow  the  two  streams  to  pass  through 
it  in  the  manner  described,  it  is  by  no  means  unusual  to 
find  membranes  which  offer  opposition  to  the  passage  of 
the  substances  which  are  dissolved ;  indeed,  the  latter  is  in 
some  cases  unable  to  pass  through  at  all,  the  membrane 
being  impermeable  to  it.  Such  a  membrane  is  called  a 
semipermeable  membrane  as  far  as  that  particular  salt  is 
concerned,  as  it  will  allow  the  water  to  pass  but  not  the 
solution  on  the  other  side.  Such  a  membrane  is  the  pre- 
cipitation membrane  produced  by  the  contact  of  aqueous 
solutions  of  potassic  ferrocyanide  and  copper  sulphate. 
It  is  difficult  to  prepare  such  a  membrane  for  experiment, 
but  it  becomes  possible  if  a  porous  pot  containing  one  of 
the  solutions  is  immersed  nearly  to  the  top  in  a  vessel 
containing  the  other.  As  the  latter  slowly  penetrates  the 
sides  of  the  porous  pot  it  eventually  comes  into  contact 
with  the  solution  in  its  interior,  and  a  precipitation  membrane 
is  formed  in  the  substance  of  the  walls  of  the  pot.  If  such 
a  prepared  pot  be  emptied  of  this  solution  and  a  solution 
of  cane  sugar  be  poured  into  it,  and  the  pot  and  its  contents 
be  placed  in  water,  the  sugar  does  not  pass  out,  though  water 
continuously  passes  in.  The  membrane  is  shown  to  be  a 
semipermeable  one,  permeable  to  water,  but  not  to  sugar 
solution. 

But   the   membrane    which    is    concerned    in    osmotic 


62  VEGETABLE  PHYSIOLOGY 

phenomena  in  the  plant  differs  from  any  that  can  be  used 
in  the  laboratory  in  one  important  point,  which  modifies 
the  process  in  a  fundamental  way.  It  is  alive,  and  exercises 
an  active  control  over  the  process.  It  consists  of  a  cell- 
wall,  on  each  face  of  which  is  a  thin  film  or  layer  of  the 
living  substance.  The  latter  is  more  complex  than  the 
ordinary  semipermeable  membrane,  for  it  can  vary  its 
behaviour  from  time  to  time  and  so  regulate  the  entry  of 
substances  both  liquid  and  dissolved,  determining  not  only 
what  shall  pass  through,  but  to  a  large  extent  in  what  con- 
centration they  shall  pass.  In  this  way  the  absorption 


FIG.  51.  —VEGETABLE  CELLS. 

A,  very  young ;    B,  a  little  older,  showing  commencing  formation  of  vacuole. 
p,  protoplasm  ;   n,  nucleus  ;   v,  a  vacuole. 

of  a  particular  salt  by  the  vegetable  cell  is  not  necessarily 
in  the  proportion  of  its  osmotic  pressure. 

We  can  apply  the  osmotic  process  to  explain  the  original 
formation  of  the  vacuole.  Consider  the  case  of  a  young 
non-cuticularised  cell  of  the  external  layer  of  a  plant  which 
is  immersed  in  water.  It  is  full  of  protoplasm,  and  limited 
or  clothed  by  a  cell-membrane  which  is  permeable  more  or 
less  readily  by  water.  The  protoplasm  is  saturated  with 
water,  but  there  is  no  separate  accumulation  of  the  latter 
in  its  interior.  Part,  at  least,  of  the  cell- wall  is  in  contact 
with  water  on  the  outside.  The  protoplasm  is  actively 
living,  and  in  the  course  of  the  chemical  changes  which 
are  incidental  to  vital  action  certain  substances  are  produced 
by  it,  which,  like  the  syrup  in  the  experiment  first  described, 


EELATION  OF  WATEK  TO  THE  PBOTOPLASM    63 


exert  a  fairly  high  osmotic  pressure.  Water  consequently 
passes  into  the  cell,  at  first  only  in  such  quantities  as  to 
distend  it  somewhat.  As  the  process  goes  on,  more  liquid 
is  taken  up  than  can  be  stored  in  the  molecular  interstices 
of  the  protoplasm.  Drops  consequently  appear,  and  these 
gradually  run  together  until  a  distinct  though  small  vacuole, 
and  later  a  number  of  such  vacuoles,  are  apparent  in  the 
protoplasm  (fig.  51).  These  soon  run  together  as  the 
amount  of  water  still  increases,  while  the  gradually  in- 
creasing hydrostatic  pressure  stretches  the  extensible  cell- 
wall  and  so  enlarges  the  cavity.  The 
growth  of  the  protoplasm  does  not 
keep  pace  with  this  extension  of  the 
wall,  and  therefore  after  a  time  the 
protoplasm  forms  a  layer  round  the 
cell-wall,  enclosing  a  single  large 
cavity  in  which  the  surplus  liquid  is 
held  (fig.  52),  the  hydrostatic  pressure 
of  the  latter  pressing  the  living  sub- 
tance  against  the  wall. 

Not  only  does  the  protoplasm 
regulate  the  entry  of  substances  into 
the  cell,  but  it  prevents  their  escape 
by  an  exosmotic  flow.  We  may 
think  of  it  as  a  semipermeable  mem- 
brane as  far  as  the  organic  contents 
of  the  cell  are  concerned,  though  it  is 

certainly  much  more  complex  than  the  term  suggests.  A 
simple  experiment  will  illustrate  this  point.  Take  a  cell 
of  the  coloured  cortex  of  the  root  of  the  beet  and  put 
it  into  contact  with  a  solution  of  higher  osmotic  pressure 
than  that  which  is  contained  in  its  own  vacuole ;  for 
instance,  a  solution  of  common  salt  of  about  10  per  cent, 
concentration.  Watch  its  action  on  a  slide  under  the 
microscope.  As  the  salt  solution  reaches  the  cell,  the 
protoplasm  of  the  latter  gradually  retreats  from  the  walls 
(fig.  53),  at  first  at  the  corners  and  then  all  round  the  sides, 


Fia.  52. — ADULT  VEGET- 
ABLE CELLS.  x  500. 
(After  Sach*.) 

h,  cell- wall ;  p,  protoplasm  ; 
k  k',  nucleus,  with  nu- 
cleoli ;  s  s',  vacuoles. 


64  VEGETABLE  PHYSIOLOGY 

till  it  appears  as  a  rounded  or  irregular  mass  in  the  centre, 
still  coloured  red.  The  salt  solution  has  abstracted  the 
water  from  the  vacuole,  and  the  protoplasm,  relieved  of 
the  pressure  outwards  caused  by  the  liquid  in  the  latter, 
has  shrunk  away  from  the  walls.  The  membrane  shows 
its  complex  character,  for  the  living  protoplasm  on  each 
side  of  the  non-living  cell-wall  becomes  separated  from  it. 
But  it  will  be  seen  that  the  colour  will  not  penetrate  the 
protoplasm,  which  in  fact  retreats  before  the  salt  solution, 


a  b 

FIG.  63. — CELLS  OF  PARENCHYMA  UNDERGOING  PLASMOLYSIS. 

a,  b,  e,  d  represent  successive  stages.     The  dotted  area  in  each  cell 

represents  the  protoplasm. 

holding  the  colouring  matter  of  the  vacuole  in  its  own 
substance.  If  now  the  salt  solution  is  replaced  by  water, 
the  latter  gradually  passes  again,  of  course  osmotically, 
into  the  cell.  It  passes  through  the  whole  thickness  of 
the  protoplasm,  the  vacuole  is  re-established,  and  the 
protoplasm  again  comes  to  line  the  cell-wall,  pressed  against 
it  by  the  water.  Not  only  did  the  protoplasm  prevent  the 
colouring  matter  from  escaping,  but  it  also  retained  the 
osmotic  substances  of  the  sap. 

The  protoplasm  thus  can  oppose  the  passage  through  it 
of  various  soluble  bodies  with  which  it  may  be  brought 
into  contact,  though  it  allows  the  water  in  which  they  are 
dissolved  to  permeate  it  freely.  In  the  experiment  just 


BELAT10N  OF  WATEK  TO  THE  PBOTOPLASM  65 

described  the  strong  salt  solution  failed  to  pass  through 
the  protoplasmic  layer  ;  the  re-entry  of  the  water  into 
the  vacuole  showed  that  the  protoplasm  prevented  the 
osmotic  substances,  originally  present  in  the  water  which 
the  cell  contained,  from  escaping  in  the  issuing  stream. 
These  substances  must  have  been  left  behind,  or  there 
would  have  been  no  osmotically  active  material  to  draw 
the  water  back,  when  it  was  allowed  to  replace  the  salt 
solution  outside  the  cell. 

That  this  behaviour  is  dependent  on  the  vital  activity 
of  the  protoplasm  can  be  shown  by  repeating  the  experi- 
ment after  killing  the  living  substance  by  a  short  immer- 
sion of  the  cell  in  alcohol.  Then  the  process  of  osmosis 
goes  on  exactly  as  in  the  first  experiment  described.  The 
salt  solution  penetrates  into  the  vacuole  as  if  only  a  cellulose 
septum  were  present,  the  dead  protoplasm  exerting  no  regu- 
lating influence. 

We  must  not  conclude  from  this  experiment  that  inor- 
ganic salts  in  all  degrees  of  concentration  are  kept  from 
entering  the  cell  by  the  protoplasm.  If  extremely  dilute 
solutions  are  employed,  the  living  substance  permits  their 
passage  together  with  a  certain  appropriate  amount  of 
water.  Similarly,  extremely  dilute  solutions  of  bodies 
found  in  the  fluid  of  the  vacuoles,  the  so-called  cell-sap, 
can  make  their  way  out  of  the  cells.  The  protoplasm 
exerts  a  definite  regulating  influence,  however,  upon  both 
the  entry  and  the  escape  of  these  different  substances.  We 
shall  have  occasion  to  discuss  this  more  fully  later. 

The  regulated  osmosis  which  is  thus  the  mode  of  entry 
of  water  into  a  cell  containing  no  vacuole,  and  which 
causes  the  growth  or  completion  of  the  vacuole,  after  its 
first  appearance,  continues  after  its  formation  is  finished. 
This  can  be  studied  most  favourably  in  aggregations  of 
cells,  such  as  we  find  in  the  cortex  of  a  stem  or  the  loose 
mesophyll  of  a  leaf,  as  in  such  cells  there  is  a  more  evident 
renewal  of  the  water  of  the  vacuoles  than  in  those  of  tissues 
which  are  surrounded  by  liquid. 

5 


66  VEGETABLE  PHYSIOLOGY 

In  such  tissues  as  those  just  mentioned  we  can  demon* 
strate  with  ease  what  is  more  difficult  to  detect  in  the 
others,  that  not  only  is  water  admitted  to  the  cells,  but  it 
is  also  given  off  from  them.  This  does  not  depend  on 
osmosis  in  the  stem  or  leaf,  but  is  due  to  evaporation, 
which  takes  place  from  the  surfaces  of  the  cells  abutting 
on  the  intercellular  spaces,  whence  the  watery  vapour  is 
exhaled  through  the  stomata,  or,  in  the  case  of  a  woody 
stem,  through  the  lenticels.  In  a  cell  surrounded  by  water 
such  removal  must  depend  upon  osmotic  currents. 

This  removal  of  w.ater  occasions  a  need  for  a  continuous 
replenishment  of  the  liquid  in  the  vacuoles,  which  is 
brought  about  by  the  same  modified  osmosis  which  has 
been  described.  We  can  see  that  this  process  must  be 
continually  taking  place  in  a  complex  of  succulent  cells. 
If  we  consider  two  which  are  contiguous  and  are  separated 
from  each  other  by  a  common  cell-wall,  it  is  evident  that 
unless  the  proportion  of  water  to  dissolved  substances  in  the 
vacuoles  of  both  is  the  same,  a  flow  of  water  from  one 
to  the  other  will  take  place  till  this  equilibrium  is  reached. 
Any  disturbance  taking  place  in  one  cell  of  a  complex  will 
hence  spread  from  cell  to  cell  until  the  composition  of  the 
fluid  contents  of  them  all  is  uniform.  When  we  consider 
the  differences,  sometimes  very  slight,  sometimes  more 
extensive,  which  are  continually  taking  place  in  the  meta- 
bolic activities  of  the  separate  cells  of  a  community,  it  is 
evident  that,  so  long  as  life  lasts,  currents  of  this  kind 
must  be  continually  passing  from  cell  to  cell  in  various 
directions,  and  frequently  at  very  different  rates. 

Evaporation  from  a  cell  into  an  intercellular  space 
must  lead  to  a  certain  increase  of  the  concentration  of  the 
solution  of  osmotically  active  substance  in  its  vacuole. 
This  then  attracts  water  from  the  contiguous  cells,  and 
consequently,  independently  of  metabolic  changes  affecting 
the  quantities  of  such  osmotic  substances,  evaporation  itself 
must  help  in  causing  movements  of  water  from  cell  to  cell. 

The    quantity   of    these   osmotic    substances   which   are 


RELATION  OF  WATER  TO  THE  PROTOPLASM     67 

present  in  any  particular  cell  will  depend  upon  the 
behaviour  of  the  protoplasm  from  time  to  time.  Such 
substances  are  usually  being  continually  produced  in  all 
growing  cells,  and  in  most  others  in  which  chemical 
changes  are  proceeding.  Hence  such  cells  are  continually 
absorbing  water,  and  are  consequently  so  full  that  a  con- 
siderable stretching  force  is  exerted  on  the  cell-wall  which 
bounds  them.  Cells  in  such  a  condition  are  called  turgid, 
and  the  condition  itself  is  known  as  turgor  or  turgescence. 
The  equilibrium  which  is  attained  by  such  a  cell  is  reached 
when  the  distension  caused  by  the  entering  osmotic  stream 
is  balanced  by  the  elastic  recoil  of  the  extensible  cellulose 
wall.  In  some  cases  the  tension  set  up  in  a  tissue  by  the 
turgescence  of  the  cells  is  sufficient  to  force  the  water,  by  a 
process  of  filtration,  through  the  walls  of  the  outermost 
ones,  so  that  it  escapes  in  drops  or  in  a  slow  stream.  This 
may  often  be  seen  on  the  edges  or  apices  of  blades  of  grass 
in  the  early  morning.  It  is  of  great  use  also  in  forcing 
water  into  the  axial  woody  cylinder  of  roots,  as  will  appear 
later.  Occasionally  the  turgescence  becomes  so  great  as  to 
lead  to  rupture  of  the  cell-walls,  as  is  the  case  in  some 
pollen  grains,  and  sometimes  in  certain  fleshy  fruits. 

That  the  condition  of  turgescence  in  cells  is  attended  by 
a  stretching  of  the  cell-walls  can  be  seen  by  taking  a  piece 
of  a  plant  which  is  turgid,  such  as  the  stalk  of  a  rhubarb 
leaf,  and,  after  carefully  measuring  its  dimensions,  steeping 
it  for  some  time  in  a  ten  per  cent,  solution  of  common  salt. 
On  removing  it,  it  will  be  found  to  have  become  flaccid, 
and  a  remeasurement  will  show  that  both  its  length  and 
thickness  have  diminished.  Turgescence  is  not,  however, 
due  simply  to  physical  causes  ;  the  protoplasm  which  lines 
the  cell  has  a  regulating  influence  over  the  passage  of  the 
water  into  and  out  of  them.  When  a  turgid  pulvinus  of 
such  a  plant  as  Robinia  or  Mimosa  is  stimulated  by  rough 
handling  of  the  leaf,  the  latter  falls  backward  from  its 
expanded  position,  and  the  fall  is  found  to  be  due  to  the 
escape  of  water  from  the  cells  of  the  lower  side  of  the  pulvinus. 

5* 


68  VEGETABLE  PHYSIOLOGY 

The  original  state  of  equilibrium  has  been  disturbed  by  the 
shook  to  the  protoplasm  administered  by  the  stimulation, 
and  the  latter  allows  or  compels  the  water  to  pass  outwards. 
The  active  influence  of   the  protoplasm  is  seen  also  in 
another  class  of  phenomena.     Certain  structures  known  as 
nectaries  occur  conspicuously  in  many  flowers.     They  are 
aggregations  of  cells  of  a  particular  kind  which  exude  a 
sugary  fluid  upon  their  surface.     The  liquid  in  the  cells 
contains  a  little  sugar,  and  this  weak  solution  is  capable 
of  passing  through  the  protoplasm,  not  by  osmosis,  but  by 
a  kind  of  nitration.     Its  concentration  is  usually  increased 
by  subsequent  evaporation  of  the  water  in  which  it  is  dis- 
solved, so  that  the  secretion  when  collected  has  a  distinctly 
sweet  taste.    When  the  petals  of  certain  flowers  bearing 
these  nectaries  are  cut  off,  and  their  cut  ends  immersed  in 
water,  the  glands  continue  for  some  time  to  exude  the 
nectar.     There  can  be  no  question  here  of  a  gross  filtration 
of  water  under  pressure  through  the  tissue,  as  there  is  no 
such  pressure  acting  on  the  base  of  the  cut  petal.     The 
protoplasm  causes  a  stream  of  water  to  flow  into  the  cells 
of  the  gland  by  producing  osmotic  substances  inside  them, 
in  this  case  chiefly  sugar.     The  turgescence  thus  set  up  in 
the  gland  cells  exerts  a  strong  hydrostatic  pressure  on  the 
limiting  membranes  of  these  secreting  cells,  which  ultimately 
so  stimulates  the  protoplasm  as  to  cause  it  to  allow  the 
sugary  solution  to  exude  upon  their  free  surfaces.     We  can 
discriminate  between  two  forces  at  work  in  the  excretion  of 
the  nectar.     The  absorption  of  water  by  the  gland  cells  is 
due  to  osmosis  ;  the  excretion  from  them  on  to  the  exterior 
of  the  gland  is  more  a  question  of  a  modified  filtration 
under  pressure  from  the  turgid  cell.     This  is  shown  by  the 
fact  that  if  the  surface  of  the  gland  is  carefully  dried,  the 
exudation  shortly  recommences.     Osmosis  is  not  possible 
under  these  conditions.     If  the  gland  is  killed  by  alcohol, 
the  sugar  already  there  is  retained  in  the  cells,  and  no 
exudation  of  nectar,  or  even  of  water,  takes  place. 

The  vital  activity  of   the  protoplasm  is  thus  seen  to  be 


RELATION  OF  WATEE  TO  THE  PROTOPLASM     69 

intimately  connected  with  the  presence  of  water  in  its 
substance.  The  importance  of  the  ready  access  of  the 
latter  is  seen  further  from  other  considerations.  We  have 
incidentally  alluded  more  than  once  to  the  fact  that  the 
liquid  concerned  in  these  osmotic  currents  is  not  pure 
water  only,  but  should  rather  be  regarded  as  an  extremely 
dilute  solution  of  various  salts,  &c.  Though  the  protoplasm 
opposes  the  passage  of  anything  like  a  strong  solution  of 
inorganic  salts,  it  allows  very  dilute  ones  to  enter  the  cell, 
much  as  it  does  pure  water.  In  this  way  the  slowly  diffusing 
stream  brings  to  the  protoplasm  of  each  cell  the  inorganic 
materials  which  are  absorbed  from  the  earth,  and  enables 
the  matters  elaborated  or  formed  from  them  by  the  proto- 
plasm to  pass  from  cell  to  cell.  The  feeding  or  nutrition  of 
the  various  cells,  together  with  the  construction  of  the  sub- 
stances which  minister  to  that  nutrition,  is  thus  dependent  on 
the  transit  of  fluid  about  the  plant  in  the  way  described.  The 
access  of  various  gases  is  similarly  made  possible,  for  these 
are  dissolved  in  the  liquid  stream.  The  oxygen  upon  the 
presence  of  which  life  depends  is  thus  transported  to  each 
cell,  and  the  carbon  dioxide  of  respiration  is  removed  from 
the  seats  of  its  liberation. 

The  condition  of  turgescence  is  necessary  also  for  growth, 
and  for  various  movements  of  different  parts,  enabling 
them  to  adapt  themselves  to  varying  conditions  of  their 
environment.  Some  plants,  particularly  those  which  are 
aquatic  in  habit,  and  such  parts  of  terrestrial  plants  as 
contain  but  little  woody  tissue,  are  dependent  on  the  tur- 
gescence of  their  cells  for  the  rigidity  which  enables  them 
to  maintain  their  position  in  the  medium  in  which  they 
live.  The  maintenance  of  the  turgid  condition  of  the  cells 
is  further  of  the  highest  importance  in  enabling  the  inter- 
change of  water  between  contiguous  cells  to  take  place  as 
freely  as  possible,  and  without  intermission.  Flaccid  cells 
do  not  effect  such  interchange  with  sufficient  readiness. 
Flaccidity  of  an  organ  is  attended  by  a  partial  collapse  of  the 
tissue,  which  involves  a  diminution  of  the  volume  of  its 


70  VEGETABLE  PHYSIOLOGY 

intercellular  spaces,  and  hence  often  a  serious  interference 
with  its  processes  of  gaseous  interchange,  particularly 
respiration.  Nor  is  the  protoplasm  unaffected  by  the 
flaccidity,  for  its  health  is  in  a  certain  degree  dependent 
upon  its  being  subjected  to  hydrostatic  pressure  by  the 
water  of  the  vacuole. 

The  importance  of  the  water  supply,  and  indeed  its 
necessity  to  the  plant,  explains  the  existence  of  certain 
subsidiary  mechanisms  for  its  absorption  and  storage 
which  are  occasionally  met  with.  These  will  be  considered 
in  detail  in  a  subsequent  chapter,  but  a  few  of  such  adapta- 
tions may  be  noticed  here.  We  frequently  find  particular 
aggregations  of  cells  set  apart  for  storage  of  water.  The 
epidermis  of  certain  parts  frequently  subserves  this  purpose, 
and  many  plants  possess  a  considerable  development  of 
aqueous  tissue,  variously  disposed,  which  forms  a  similar 
storehouse.  The  cells  of  this  tissue  contain  little  else  than 
water,  and  thus  serve  to  supplement  the  vacuoles  of  the 
ordinary  cells.  In  plants  that  inhabit  dry,  arid  soils  such 
as  sandy  deserts  there  are  often  other  adaptations  relating 
to  water  storage.  Such  plants  are  often  covered  with  large 
bladder-like  hairs  which  hold  a  considerable  quantity  of 
liquid.  Plants  which  are  exposed  to  conditions  threatening 
too  copious  evaporation  are  generally  furnished  with  a  very 
prominent  cuticle  tending  to  check  undue  escape. 


71 


CHAPTER  V 

THE    TRANSPORT    OF   WATER   IN    THE    PLANT 

We  have  seen  that  it  is  necessary  for  the  life  of  a  plant 
that  all  its  living  cells  shall  be  freely  supplied  with  water. 
According  to  the  habit  of  life  of  plants  the  mode  of  supply 
must  necessarily  vary.  Those  which  are  so  constituted 
that  water  finds  free  access  to  all  the  cells,  such  as  the 
unicellular  or  filamentous  Algce,  which  live  in  streams,  pools, 
&c.,  present  no  difficulty,  as  osmosis  can  go  on  freely  in  each 
cell,  water  entering  its  vacuole  from  the  exterior.  Sturdier 
plants  of  aquatic  habit  are  almost  equally  easily  supplied  ; 
the  water  enters  by  osmosis  into  the  vacuoles  of  the  epider- 
mal cells,  the  walls  of  which  in  these  plants  are  not  cuticu- 
larised,  and  from  them  it  can  pass  from  cell  to  cell  all  over 
the  plant-body.  No  force  in  addition  to  osmosis  is  necessary 
in  these  undifferentiated  plants.  But  the  great  number  of 
plants  which  have  a  terrestrial  habitat,  from  the  nature 
of  their  environment  require  a  more  elaborate  mechanism. 
This  is  found,  as  we  have  already  pointed  out,  in  the  well- 
differentiated  system  of  conducting  tissue,  composed  largely  of 
lignified  vessels,  fibres,  and  cells.  Throughout  all  such  plants 
a  stream  of  water  passes,  entering  at  the  roots,  passing  along 
the  woody  axis,  and  so  rising  up  the  stem  into  the  leaves, 
where  a  very  large  part  of  it  is  evaporated.  This  stream  of 
water  is  often  known  as  the  ascending  sap.  In  addition 
to  this  comparatively  rapid  stream,  slow  currents  of  diffusion 
from  cell  to  cell  are  also  maintained,  as  in  the  plants  of 
humbler  type.  These  diffusion  currents,  depending  mainly 


72  VEGETABLE  PHYSIOLOGY 

on  osmosis  between  contiguous  cells,  have  not  the  definite 
direction  of  the  rapid  current,  and  play  quite  a  subordinate 
part  in  the  supply  of  the  whole  plant  with  water.  They 
are,  however,  supplementary  to  the  ascending  sap,  and  effect 
interchanges  in  regions  which  the  latter  does  not  immediately 
reach.  The  cortex  of  the  axis  of  the  plant  is  especially 
dependent  upon  them,  as  various  mechanisms  exist  in  the 
different  regions  of  the  stele  to  guard  against  too  free  an 
escape  of  Water  from  its  tissues  into  the  cortex. 

Except  in  some  special  cases  the  water  which  passes 
through  the  body  of  an  ordinary  terrestrial  plant  is  obtained 
from  the  soil  in  which  its  roots  are  embedded.  The  soil 
itself  is  composed  of  minute  particles  of  inorganic  matter  of 
very  different  degrees  of  solubility,  derived  originally  from 
the  breaking  down  of  rocks,  together  with  decaying  animal 
or  vegetable  matter  mixed  with  the  inorganic  constituents. 
This  organic  matter  is  known  as  humus  and  is  of  very  varied 
composition.  The  soil  thus  consists  of  a  loose  matrix  of 
granular  character,  the  interspaces  of  which  are  normally 
filled  with  air.  The  air  is  in  most  cases  mixed  with  a  certain 
quantity  of  carbon  dioxide  which  is  being  evolved  from  the 
humus  constituents  of  the  soil,  and  which  is  slowly  exhaled 
from  the  surface.  The  interspaces  are  capable  of  con- 
taining varying  quantities  of  water  ;  indeed  the  soil  may  be 
so  saturated  with  it  that  they  are  all  full.  We  find  soils  of 
all  conditions  in  this  respect,  from  the  dry  sands  of  deserts 
to  the  mud  of  bogs.  The  water  may  be  held  with  greater 
or  less  tenacity,  clays  and  sandy  soils  affording  instances  of 
two  extremes  in  that  particular.  When  the  interspaces  of 
the  soil  are  filled  with  water,  the  plants  which  it  is  support- 
ing are  very  unfavourably  placed  for  absorbing  the  liquid. 
By  the  excess  of  water  their  roots  are  deprived  of  the  air 
which  they  need  for  purposes  of  respiration  ;  their  structure 
does  not  enable  the  absorption  of  water  to  take  place  all  over 
their  surfaces,  as  their  external  cells  are  more  or  less  cuti- 
cularised  ;  they  are  consequently  hindered  and  not  helped  by 
the  superfluity  of  liquid.  When  a  soil  is  properly  drained,  its 


THE  TRANSPOKT  OF  WATER  IN  THE  PLANT  73 

interspaces  are  filled  with  air,  and  a  delicate  film  of  water 
surrounds  each  of  its  particles  and  adheres  closely  to  it. 
This  water,  often  spoken  of  as  hygroscopic  water,  is  the 
source  of  the  plant's  supply.  The  presence  of  air  in  the 
interspaces  supplies  the  wants  of  the  root  and  frees  it  from 
the  difficulties  which  have  been  pointed  out. 

The  hygroscopic  water  adheres  so  closely  to  the  particles 
of  the  soil  that  it  escapes  ordinary  observation  ;  when, 
however,  soil  that  has  been  allowed  to  dry  at  any  ordinary 
temperature  till  its  interspaces  are  apparently  empty,  is 
exposed  to  a  heat  approaching  that  of  boiling  water,  a 
considerable  quantity  of  vapour  is  given  off,  due  to  the 
volatilising  of  the  hygroscopic  films. 

The  difficulty  of  the  entry  of  the  water  into  the  cells 
of  the  outermost  layers  of  the  young  roots  involves  the 
development  of  a  special  absorptive 
mechanism  upon  them.  This  takes  the 
form  of  a  number  of  delicate  outgrowths 
of  the  internal  cells,  which  form  long 
thin-walled  hairs  (fig.  54).  These  are 
not  distributed  all  over  the  surface  of  the 
young  rootlets,  but  are  confined  to  a 
particular  region  not  far  behind  the 
apex.  As  the  delicate  branches  of  the 
root  grow,  the  root-hairs  farthest  from 
the  tip  gradually  perish,  more  being 
formed  continually  at  about  the  same 
distance  from  the  apex.  There  is  thus 

x  FIG.    54—  ULTIMATE 

a   continuous  renewal  of  this  collection     BRANCHES    OF   A 


of   hairs,   which   is   maintained   as   long     RoOT' 

POSITION  OF  ROOT- 

as  the  root  system  extends  and  continues      HAIRS. 
functional.     The  interspaces  of  the  soil 
are  penetrated  by  the  young  roots,  the  manner  of  whose 
growth  involves  a  very  close  approximation  of  their  sub- 
stance to  the  surface  of  the  particles  of  which  the  soil  con- 
sists.    The  delicate  hairs  standing  out  at  right  angles  to 
the  surface  of  the  roots  are  consequently  brought  into  very 


74 


VEGETABLE  PHYSIOLOGY 


close  and  intimate  relations  with  these  particles  and  with 
the  film  of  hygroscopic  water  which  surrounds  them.  In 
some  cases  the  pressure  between  the  two  is  so  close  that 
the  particles  become  embedded  in  the  membrane  (fig.  55). 
The  hygroscopic  film  of  water  is  thus  separated  from  the 
interior  of  the  root-hair  by  a  most  delicate  pellicle  of  cell- 
wall  substance,  lined  by  an  almost  equally  delicate  layer  of 
protoplasm.  The  vacuole  of  the  hair  contains  a  somewhat 
acid  cell-sap,  the  acidity  being  due  to  the  presence  generally 
of  acid  potassium  phosphate,  by  virtue  of  which  osmosis  is 
set  up  ;  the  osmotic  pressure  of  the 
sap  being  considerable,  the  cell  quickly 
becomes  turgid  and  distended,  such 
turgescence  continuing  so  long  as  the 
conditions  remain  favourable.  The  root- 
hairs  are  very  numerous,  and  their  united 
action  causes  a  considerable  accumula- 
tion of  water  in  the  cortex  of  the  root, 
for  it  passes  into  the  cells  of  this  region 
by  osmosis  through  the  base  of  the  hair. 
This,  being  one  of  the  cells  of  the  ex- 
ternal layer,  impinges  upon  one  or  more 
of  the  cortical  cells,  which  have  a  similar 
reaction  to  that  of  the  root-hair  itself. 
FIG.  55.—  ROOT-HAIR  Osmotic  currents  are  thus  set  up  from 
eyery  }}&{?  and  a  gradual  accumulation 

J 

of  water  takes  place  in  the  cortex  of  the 
young  root,  making  all  its  cells  turgescent 
and  causing  a  considerable  hydrostatic  pressure  in  the 
tissue.  This  turgescence  with  its  consequent  pressure  soon 
extends  all  along  the  axis  of  the  young  root,  though  it  is 
originally  set  up  only  by  the  region  which  is  clothed  by 
the  absorbing  hairs. 

The  central  portion  of  the  axis  of  the  root  is  occupied 
by  a  cylindrical  mass  which  extends  throughout  its  whole 
length,  and  which  is  known  as  the  stele  (fig.  56).  It  is 
generally  marked  off  sharply  from  the  cortex,  the  cells  of 


IN  CONTACT   WITH 

PARTICLES  OF  SOIL. 

x  300. 


THE  TEANSPOET  OF  WATEE  IN  THE  PLANT  75 

whose  innermost  layer,  the  endodermis,  are  often  peculiarly 
thickened,  in  much  the  same  manner  as  those  of  the 
exodermis  already  described  (page  25).  This  thickening  is  not, 
however,  usually  very  marked  in  the  region  of  absorption. 
At  certain  places  round  the  periphery  of  the  stele  of  the  root, 
the  woody  strands  (fig.  56,  Sp)  may  be  seen.  These  are  in 
contact  with  the  succulent  and  turgid  parenchyma  which  has 
been  filled  with  water  in  the  way  described,  and  consequently 
the  hydrostatic  pressure  which  has  thus  been  set  up  is'Jbrought 


Fiet.  56. — SECTION  OF  ROOT,  SHOWING  ROOT-HAIRS  ABUTTING  ON  THK 
PARENCHYMA  OF  THE  CORTEX,  AND  THE  WOODY  STRANDS,  Sp,  OF 
THE  STELE.  (After  Kny.) 

to  bear  upon  the  walls  of  the  woody  vessels  which  constitute 
the  greater  part  of  those  strands.  These  form  the  lower 
portions  of  continuous  open,  or  nearly  open,  tubes,  which 
extend  from  the  roots  to  the  leaves  ;  at  the  time  when  the 
absorption  of  the  root-hairs  and  cortex  is  greatest  these 
vessels  are  empty,  or  nearly  so,  and  the  effect  of  the  hydro- 
static pressure  on  their  walls  is  to  force  the  water  from  the 
turgid  cortex  into  the  walls  and  cavities  of  the  vessels. 
How  the  water  is  distributed  is  not  fully  known ;  we  have 
seen  that  lignified  cell- walls  have  a  certain  power  of  taking 
up  water,  and  of  passing  it  on  with  considerable  rapidity, 
so  that  part  of  it  may  be  expected  to  remain  in  the  walls. 
Part,  however,  passes  through  into  the  cavities  of  the  vessels, 


76 


VEGETABLE  PHYSIOLOGY 


and  in  the  early  part  of  the  year,  before  the  leaves  of  the 
plant  expand,  they  thus  become  filled  with  liquid.  This 
filtration  into  the  vessels  tends  to  relieve  the  pressure  in  the 
cortex,  and  additional  water  can  then  be  absorbed  from  the 
soil  as  before.  The  consequent  increase  of  the  turgescence  is 
followed  by  further  filtration  into  the  vessels,  and  these 
two  factors  continually  acting  together,  the  water  is  made 
to  rise  gradually  in  the  axial  stele.  The  root-hairs  and 
the  turgid  cortex,  in  fact,  exert  in  this  way  a  kind  of  con- 
tinuous pumping  action,  forcing  it  along  the  axis.  The 
force,  which  is  the  expression  of  the  elastic  recoil  of  the 
cell-walls  of  the  over- distended  cortical  cells,  and  which 

is  brought  to  bear  upon  their 
fluid  contents,  squeezing  a  quan- 
tity of  liquid  through  the  cell- 
walls  into  the  vessels,  is  known 
as  root -pressure,  and  is  one  of 
the  main  factors  in  the  transport 
of  water  through  the  plant. 

The  turgescence  not  only 
leads  to  the  rise  of  the 
sap  in  the  axial  stele,  but  it 
spreads  throughout  the  whole  of 
the  cortical  tissue  of  the  plant, 
stem  as  well  as  root,  reaching, 
indeed,  every  cell  into  which 
osmotic  transport  can  take  place. 
The  action  of  the  root-hairs  is 
thus  responsible  not  only  for  the 
rapid  ascent  of  the  sap,  but  also 
for  the  maintenance  of  turgidity 
outside  the  region  supplied  by  the  ascending  stream. 

The  stele  of  the  root  is  directly  continuous  with  that  of 
the  stem,  and  though  the  disposition  of  the  woody  elements 
is  somewhat  different  in  the  two  regions,  there  is  no  doubt 
that  they  also  are  continuous  throughout  (fig.  57).  The 
stream  of  water  consequently' passes  up  the  woody  tissue  of 


Fio.     67. — DIAGRAM 
COURSE    OF    THE 
BUNDLES    IN     A 
DONOUS  PLANT. 


SHOWING 

VASCULAR 

DlCOTYLE- 


THE  TRANSPORT  OF  WATEB  IN  THE  PLANT  77 


the  stem  so  long  as  the  cells  are  living.  The  stream  in 
young  plants  passes  along  the  whole  substance  of  the  wood, 
which  in  most  cases  forms  a  central  mass  of  some  size. 
In  herbaceous  plants  the  bundles  do  not  usually  form  a 
continuous  cylinder,  but  are  more  or  less  isolated  in  their 
course.  In  old  trees  the  water-conducting  area  is  limited 
to  the  outer  regions  of  the  central  woody  mass,  which  are 
known  as  the  alburnum  or  sap-wood.  The  central  portion 
of  the  wood  is  dead,  and  the  cell-walls  are  often  very  much 
altered  in  chemical  composition.  This  region  is  known 
as  the  duramen  or  heart-wood  ;  it  takes  no  part  in  the 
conduction,  the  tissue  always  remaining  dry. 

The  vascular  bundles  are  seen  to  be  continuous  from 
the  axis  to  the  leaves,  where  they  are  no  longer  found 
arranged  in  a  cylindrical  manner,  but  are  disposed  in 
various  ways  as  a  much-branched  net- 
work (fig.  58).  The  separate  ramifica- 
tions are  known  technically  as  veins, 
and  they  are  distributed  in  the  various 
ways  known,  largely  through  the  method 
of  branching  of  the  leaf  axis.  The 
latter,  however,  with  very  rare  excep- 
tions, is  flattened  or  winged  throughout 
the  whole  or  part  of  its  length,  and  the 
wings  or  flattened  portions  are  supplied 
with  veins  continuous  with  those  of 
the  branched  or  unbranched  axis.  The 
vascular  tissue,  therefore,  if  traced  from  below  upwards,  is 
seen  to  exhibit  a  separation  of  its  constituent  bundles,  which 
continually  appear  to  subdivide  until  they  form  a  series  of 
delicate  ramifications  of  considerable  tenuity  which  per- 
meate the  whole  of  the  flattened  portions  of  the  leaves  or 
other  parts.  The  tenuity  of  the  ultimate  endings  of  the 
vascular  bundles  is  attended  with  certain  changes  in  the 
character  of  the  constituent  cells,  but  they  remain  woody 
and  irregularly  thickened  as  they  are  lower  down  in  the 
axis.  In  the  leaves  these  endings  of  the  bundles,  which  are 


FIG.  58. — VASCULAR 
BUNDLES  (VEINS) 
OF  LEAF. 


78  VEGETABLE  PHYSIOLOGY 

sometimes  free,  and  sometimes  disposed  in  the  form  of  an 
open  network,  are  surrounded  by  delicate  parenchymatous 
tissue,  whose  cells  are  in  immediate  contact  with  the  woody 


FIG.  59. — ENDING  OF  A  FIBRO-VASCULAR  BUNDLE  IN  THE 
PARENCHYMA  OF  A  LEAF. 


elements,  as  they  are  in  the  root  (tig.  59).  These  delicate 
cells  are  also  in  contact  with  the  special  parenchyma  of  the 
leaf,  which  is  in  part  very  loosely  arranged  and  provided 
with  a  great  development  of  the  intercellular  space  system 


FIG.  60. — TRANSVERSE  SECTION  OF  THE  BLADE  OF  A  LEAF,  SHOWING 
THE  INTERCELLULAR  SPACES  OF  THE  INTERIOR.     X   100. 

(tig.  60),  which  we  have  seen  to  be  characteristic  of  the  whole 
of  the  tissue  of  the  plant.  The  cells  abutting  on  the  bundles 
are  filled,  like  the  root-hairs  and  the  cells  of  the  cortex, 
with  a  watery  sap  which  contains  substances  exerting  a 
relatively  high  osmotic  pressure.  The  woody  elements 


THE  TKANSPOKT  OF  WATEK  IN  THE  PLANT    79 

of  the  veins  are  not  completely  empty  ;  their  walls,  at  any 
rate,  are  saturated  with  the  water  ascending  from  the  roots. 
We  have  consequently  here  a  resumption  of  the  osmosis 
which  we  noticed  playing  so  conspicuous  a  part  in  the  original 
absorption  of  water.  The  water  is  drawn  from  the  woody 
elements  into  the  parenchyma  of  the  leaf,  and  as  it  passes 
from  cell  to  cell  the  leaf  tissue  is  made  turgescent.  The 
turgescence  is  very  largely  due  to  the  ascending  stream, 
whose  progress  we  have  traced  ;  at  the  same  time  we  must 
remember  that  the  turgid  cortex  of  the  root  is  continuous 
through  that  of  the  stem  with  the  soft  tissues  of  the  leaves, 
and  hence  the  slow  movement  of  diffusion  assists  in  its 
maintenance.  In  plants  which  have  but  little  woody  tissue, 
such  as  the  greater  number  of  herbaceous  annuals,  this 
slow  movement  plays  relatively  a  more  important  part  than 
in  those  trees  which  have  a  conspicuously  woody  trunk. 

As  we  have  seen,  the  turgid  mesophyll  tissue  has  a  great 
part  of  the  surface  of  its  cells  abutting  on  the  intercellular 
spaces  of  the  leaf.  The  cortical  cells  of  the  axis  are  also 
similarly  placed,  though  the  spaces  are  much  smaller  in 
that  region.  The  intercellular  spaces  of  the  plant  are  in 
communication  throughout,  and  the  cells  which  abut 
upon  them  are  in  most  places,  and  particularly  in  the 
leaves,  furnished  with  very  delicate  cell-walls,  which  readily 
allow  a  process  of  evaporation  to  take  place,  watery  vapour 
passing  into  the  passages.  The  whole  intercellular  space 
system  thus  becomes  charged  with  vapour,  the  process  of 
evaporation  from  the  cells  being,  however,  much  more 
marked  in  the  leaves,  owing  to  the  greater  development  of 
the  spaces  there.  At  particular  spots  in  the  leaves  and 
other  green  portions  of  the  plant,  these  intercellular  spaces 
or  canals  communicate  with  the  external  air  by  means  of 
small  openings  or  crevices  in  the  outer  layer  of  cells,  which 
are  known  as  stomata  (fig.  61).  Each  stoma  is  surrounded 
by  two  cells  of  peculiar  shape,  known  as  guard-cells,  which 
by  being  approximated  to  each  other  to  a  greater  or  less 
degree,  enable  the  extent  of  the  communication  to  be 


80 


VEGETABLE  PHYSIOLOGY 


varied  from  time  to  time  according  to  the  conditions  of 
the  plant.  The  ultimate  escape  of  the  watery  vapour  from 
the  interior  of  the  plant  is  subject  by  means  of  these  stomata 
to  a  very  delicate  regulation.  So  long  as  the  apertures 
are  open  the  watery  vapour  diffuses  outwards  into  the 
external  air.  We  may  thus  have  a  copious  exhalation 
taking  place  from  the  surfaces  of  the  leaves  and  other 


FIG.  61. — THREE  STOMATA  ON  THE  LOWER  SURFACE  OF  A  LEAF, 
DIFFERENT  DEGREES  OF  CLOSURE. 

green  parts,  which  plays  an  important  part  in  causing  the 
flow  of  water  through  the  plant.  This  evaporation  or 
exhalation  from  the  surface  is  known  as  transpiration  ;  it 
will  be  discussed  more  fully  in  a  subsequent  chapter. 

Little  or  no  evaporation  takes  place  from  the  surface 
of  the  epidermal  cells  of  the  leaves,  which  have  their  outer 
walls  generally  cuticularised  to  a  greater  or  less  extent,  the 
cuticle  offering  considerable  resistance  to  the  passage  of 
water  or  watery  vapour  through  them  in  either  direction. 

The  escape  of  watery  vapour  by  transpiration  is  supple- 
mented in  some  cases  by  an  actual  excretion  of  water  in 


THE  TKANSPOKT  OF  WATER  IN  THE  PLANT    81 

the  liquid  form.  This  happens  when  the  hydrostatic 
pressure  is  very  high  at  times  in  herbaceous  plants,  water 
being  forced  out  at  the  tips  of  the  leaves.  It  is  not  infre- 
quently seen  in  the  case  of  grasses,  the  edges  or  apices  of 
whose  leaf-blades  may  show  drops  of  liquid  standing  upon 
them  in  the  early  morning.  Similar  drops  are  often  to  be. 
seen  on  the  surfaces  of  the  leaves  of  Alcliemilla  when  they 
have  ceased  to  transpire  during  the  night,  while  the  absorp- 
tion of  water  by  the  root  has  continued  actively.  The 
escape  of  liquid  in  this  way  is  due  to  a  nitration  similar  to 
that  by  which  the  water  is  forced  into  the  woody  elements 
of  the  stele  of  the  root,  as  previously  described. 

A  subsidiary  mechanism  allowing  the  escape  of  watery 
vapour  from  the  cortex  of  stems  and  roots  is  provided  by 
the  lenticels.  We  have  seen  that  these  are  loose  aggrega- 
tions of  corky  cells  which  are  developed  in  connection  with 
the  sheaths  of  cork  that  form  part  of  the  secondary  tegu- 
mentary  protective  tissue  of  a  thickened  axis  (fig.  39). 
They  are  not,  however,  so  intimately  connected  with  evapora- 
tion as  the  stomata,  probably  being  more  concerned  with 
the  aeration  of  the  tissue. 

The  stream  of  water  thus  passing  through  the  plant 
has  a  very  important  influence  upon  its  development. 
We  have  seen  how  important  a  factor  in  its  growth  is  the 
maintenance  of  a  condition  of  turgescence,  which  in  turn 
depends  on  the  constant  absorption  of  water  to  take  the 
place  of  that  removed  by  evaporation.  The  quantity  pass- 
ing is  correlated  with  the  amount  of  leaf  surface  which  the 
plant  possesses  ;  where  there  is  a  large  leaf  area  there  is 
copious  transpiration  ;  this  necessitates  a  large  path  for  the 
ascending  stream,  and  a  consequent  development  of  the 
axial  portions  of  the  plant. 

The  greatest  increase  in  the  number  of  the  protoplasts 
takes  place  at  the  so-called  growing  points,  which  are  situated 
at  the  terminations  of  the  twigs,  and  which  give  rise  con- 
tinually to  additional  leaves  and  branches.  The  develop- 
ment of  new  material  of  this  kind  and  of  the  new  protoplasts 

6 


82  VEGETABLE  PHYSIOLOGY 

which  they  contain  is  largely  dependent  upon  another 
feature  of  the  water  supply  to  which  attention  has  already 
been  called.  A  considerable  part  of  the  material  from 
which  the  food  of  the  plant  is  constructed  is  absorbed  from 
the  soil  in  solution  in  the  water,  and  is  transported  by 
means  of  this  stream  to  the  regions  of  cell-formation.  The 
fact  that  the  quantity  of  the  nutritive  salts  in  the  water  is 
extremely  small  is  a  further  reason  for  the  transport  of 
such  large  quantities  of  water  as  pass  through  the  plant ; 
for  by  the  gradual  concentration  of  the  solution  in  the  cells 
of  the  leaf  enough  new  material  can  be  obtained  by  the 
protoplasts  for  the  construction  of  the  food  necessary  for 
their  nutrition,  growth,  and  multiplication.  Where  there 
is  a  large  flow  of  water,  as  in  a  tree,  there  is  a  continuous 
formation  of  new  cells  and  of  the  various  mechanisms  their 
life  demands  ;  where  the  transpiration  is  but  slight,  as  in  a 
Cactus,  or  where  the  supply  of  water  is  limited,  as  is  the 
case  with  jsuch  plants  as  grow  in  deserts  or  in  rocky  situa- 
tions, there  is  but  little  formation  of  new  substance. 


83 


CHAPTEE  VI 

THE    TEANSPIRATION    CURRENT.      ROOT-PRESSURE. 
TRANSPIRATION 

In  terrestrial  plants,  so  long  as  circumstances  are  favour- 
able to  the  vital  activity  of  the  organism,  we  have,  as  we 
have  seen,  a  stream  of  water  passing  from  the  roots  through 
the  axis  to  the  green  twigs  and  leaves,  where  the  greater 
part  of  it  is  evaporated.  The  stream,  which  we  have 
spoken  of  as  the  ascending  sap,  is  often  called  the  transpira- 
tion current.  Its  path  through  the  axis  of  the  plant  has 
been  determined  to  be  the  xylem  vessels,  which  are  in 
complete  continuity  from  the  young  rootlets  to  the  veins 
of  the  leaves. 

In  thick  tree-trunks,  in  which  the  wood  can  be  seen  to 
consist  of  alburnum  and  duramen,  the  stream  is  confined 
to  the  former.  Proof  of  this  can  be  obtained  in  various 
ways.  If  an  incision  is  made  all  round  the  trunk  of  a  tree 
and  a  ring  of  tissue  removed,  everything  being  cut  away 
down  to  the  outermost  ring  of  wood,  the  leaves  of  the  parts 
above  the  wound  continue  to  be  turgid.  If,  on  the  other 
hand,  the  woody  cylinder  is  cut  through,  while  the  con- 
tinuity of  the  cortex  and  that  of  the  pith  are  allowed  to 
remain  intact,  the  leaves  very  speedily  droop  and  become 
flaccid. 

If  a  plant  in  a  pot  is  watered  with  a  solution  of  a  dye 
which  has  no  noxious  action  on  the  protoplasts,  the  colour- 
ing matter  is  absorbed  with  the  liquid  which  the  roots  take 
up,  and  its  progress  can  be  traced  by  a  subsequent  micro- 
scopic examination  of  the  various  tissues  of  the  axis.  The 

6* 


84  VEGETABLE  PHYSIOLOGY 

colouring  matter  will  be  found  to  have  stained  the  wood 
for  a  considerable  distance  ;  in  the  case  of  a  small  plant, 
indeed,  it  will  be  coloured  quite  up  to  the  veins  of  the 
leaves,  while  the  pith  and  cortical  tissues  will  remain  un- 
stained. An  isolated  branch  can  be  taken  as  the  subject 
of  the"  experiment,  its  cut  surface  being  placed  in  a  solution 
of  the  dye. 

The  dye  in  these  cases  passes  with  the  current  of  water, 
as  may  be  seen  by  the  difference  in  its  rate  of  passage  when 
transpiration  is  vigorous,  and  when  from  severance  of  the 
leaves  of  a  branch  it  can  penetrate  only  by  diffusion. 

A  good  deal  of  controversy  has  been  excited  with  refer- 
ence to  the  manner  in  which  the  transport  of  the  water  in 
the  wood  takes  place.  Sachs  originally  suggested  that  the 
path  was  altogether  the  walls  of  the  cells,  and  that  their 
cavities  were  empty.  This  view  was  based  partly  on  the 
fact  that  the  vessels  undoubtedly  contain  a  quantity  of  air 
during  the  period  of  active  vegetation,  and  that  this  air  is 
at  a  lower  pressure  than  that  of  the  atmosphere.  Another 
reason  advanced  for  it  was  founded  on  the  nature  of  lignin 
and  its  relation  to  water.  While  refusing  to  imbibe  much 
water  and  swell  as  cellulose  can  be  made  to  do,  lignin  can 
contain  'a  certain  quantity,  which  it  will  part  with  very 
easily.  On  this  view  the  walls  of  the  lignified  vessels  may  be 
regarded  as  a  column  of  water  held  together  by  the  molecules 
or  micellae  of  lignin.  A  very  little  water  removed  from 
the  top  of  such  a  column  would  be  immediately  replaced 
from  below  so  long  as  a  supply  existed  there. 

Such  a  remarkable  conductivity,  however,  is  probably 
not  possessed  by  the  walls  of  the  vessels.  Many  observations 
made  in  recent  years  tend  to  negative  this  view,  and  to 
support  the  hypothesis  that  the  water  passes  in  the  cavities 
of  the  vessels.  Sachs's  opinion  that  these  are  always 
free  from  water  during  active  transpiration  has  been  shown 
not  to  be  well  founded,  for  various  observers  have  proved  that 
their  cavities  are  occupied  by  a  chain  of  water-columns 
and  air-bubbles,  the  air  having  been  originally  absorbed 


THE  TRANSPIKATION  CURRENT  85 

from  the  intercellular  space  system.  If  the  end  of  a  transpir- 
ing branch  is  injected  for  a  short  distance  with  a  viscid 
fluid,  which  will  penetrate  the  cavities  of  the  vessels  and 
subsequently  solidify,  these  passages  can  be  occluded  for 
a  distance  of  a  few  centimetres.  Gelatin  or  paraffin  can 
be  used  for  the  experiment,  being  injected  at  a  moderately 
low  temperature  such  as  will  not  injure  the  vitality  of  the 
tissue.  If  after  it  has  solidified  a  fresh  surface  is  made  by 
a  clean  cut  a  very  short  distance  from  the  end,  and  the 
branch  immersed  in  water,  the  leaves  very  soon  flag,  even 
if  some  pressure  is  applied  to  the  water  in  contact  with  the 
cut  surface.  If  the  path  of  the  liquid  were  the  cell- walls,  no 
obstacle  being  offered  to  the  transfer  of  water  to  them,  the 
upper  portions  ought  to  remain  turgid.  The  experiment 
shows  that  the  normal  channels  are  blocked  by  the  paraffin 
or  gelatin  used,  and  flagging  results  from  the  obstruction. 

A  similar  demonstration  that  the  water  passes  by  the 
cavities  or  lumina  of  the  cells  is  afforded  by  the  experi- 
ment of  compressing  the  stem  in  a  vice  ;  if  the  pressure  is 
carried  so  far  as  partially  or  entirely  to  obliterate  their 
cavities,  the  rate  of  flow  is  materially  interfered  with. 

The  progress  of  a  dye  injected  into  the  surface  of  a  cut 
branch  also  points  to  the  same  conclusion.  If  such  stains 
as  fuchsin  or  eosin,  which  colour  wood  very  rapidly,  are 
forced  up  into  a  stem  and  sections  made  almost  immediately, 
the  lignified  walls  will  be  found  to  be  in  process  of  staining, 
and  the  colour  will  be  seen  to  be  deepest  on  the  side  of  the 
wall  abutting  on  the  lumen,  often  only  penetrating  partly 
through  the  thickness.  If  the  wall  itself  were  the  path 
of  the  pigment  solution,  its  thickness  would  be  stained 
uniformly  as  far  as  the  dye  penetrated  at  all. 

The  rate  at  which  the  transpiration  current  naturally 
flows  varies  a  good  deal,  plants  showing  differences  among 
themselves  as  to  facilities  of  transport.  In  a  fairly  vigorous 
tree  it  may  be  taken  to  be  about  1-2  metres  per  hour,  though 
in  some  plants  it  has  been  observed  to  be  three  times  as 
rapid.  In  other  cases  as  low  a  speed  as  0-2  metre  per  hour 


86  VEGETABLE  PHYSIOLOGY 

has  been  found.  It  is  a  little  difficult  to  measure  in  most 
cases  ;  the  plan  generally  adopted  has  been  to  immerse 
the  cut  ends  of  branches  in  a  solution  of  such  a  dye  as  eosin, 
and  notice  how  far  the  dye  penetrates  in  some  unit  of  time. 
The  objection  to  this  method  is  that  very  frequently  the 
water  of  such  a  coloured  solution  will  travel  faster  than 
the  dye  dissolved  in  it.  McNab  used  instead  a  solution  of 
a  salt  of  lithium,  which  he  found  was  free  from  this  objection. 
He  detected  the  rate  of  progress  of  the  lithium  by  means  of 
spectroscopical  examination,  ascertaining  how  far  the  metal 
could  be  traced  in  the  stem  when  pieces  were  cut  out  and 
burnt  after  definite  intervals,  during  which  absorption  had 
proceeded. 

The  causes  of  the  transpiration  current  are  not  fully 
known,  but  there  is  no  doubt  that  it  is  due  to  the  co-opera- 
tion of  many  factors,  not  one  of  which  by  itself  is  sufficient 
to  account  for  it.  Two  of  the  main  influences  which  are 
at  work  have  been  incidentally  alluded  to,  which  must  now 
be  discussed  in  greater  detail.  These  are  the  constant 
pumping  action  of  the  cortex  of  the  root,  giving  us  the 
force  known  as  root-pressure,  together  with  the  evaporation 
into  the  intercellular  spaces,  and  its  exhalation  from  the 
surfaces  of  the  green  parts  of  the  plant,  which  we  have 
spoken  of  as  transpiration.  Eecent  investigations  make  it 
probable  that  we  must  add  to  these  the  force  of  osmosis  in 
the  parenchyma  of  the  leaves,  which  apparently  brings 
about  the  passage  of  the  water  from  the  veins  into  the  cells 
of  the  leaf-substance. 

Besides  these,  other  factors  have  been  held  to  co-operate, 
though  much  less  certainly  than  they.  The  walls  of  the 
vessels  having  an  extremely  narrow  calibre,  capillarity  has 
been  suggested  as  playing  a  part.  This  cannot,  however, 
have  much  effect  in  a  system  of  closed  trachei'ds,  like  those 
of  the  secondary  wood  of  the  Conifers,  which,  nevertheless, 
conduct  the  water.  It  has  been  thought  that  the  living 
cells  of  the  parenchyma,  which  abut  upon  the  woody  tissue 
of  the  stele,  may  play  a  part  similar  to  the  pumping  action 


KOOT-PKESSUKE  87 

of  the  root.  The  medullary  rays  of  the  stele  in  tall  tree 
trunks  have  been  held  to  behave  similarly.  Against  this 
theory  we  have  the  fact  that,  if  the  transpiration  current 
is  made  to  contain  substances  that  are  poisonous  to  the 
living  cells,  and  the  latter  are  consequently  killed,  the 
current  still  goes  on.  Considerable  lengths  of  a  stem  have 
been  killed  by  heating  it  to  the  temperature  of  boiling 
water,  and  the  dead  part  has  proved  to  be  no  obstacle  to 
the  transport.  Nor  do  differences  of  gaseous  pressure 
within  and  without  the  plant,  or  at  different  portions  of  the 
axis,  explain  the  matter  more  satisfactorily. 

ROOT-PRESSURE. — We  have  seen  how  the  absorption  of 
water  osmotically  from  the  soil  by  the  root-hairs  leads  to  a 
great  turgescence  of  the  tissue  of  the  cortex  of  the  root,  not 
only  in  the  regions  of  absorption  but  along  the  whole  length 
of  the  younger  portions,  which  turgescence  exerts  consider- 
able pressure  on  the  sides  of  the  vessels  and  trachei'ds  of  the 
xylem  of  the  stele.  By  this  means  water,  containing  various 
salts  and  other  constituents  in  extremely  small  quantity, 
is  forced  into  the  nbro-vascular  tissue.  The  process  is  not 
a  purely  physical  one  of  nitration  under  pressure,  but  is 
regulated  to  some  extent  by  the  protoplasm  of  the  cells  which 
abut  upon  the  xylem.  When  these  are  distended  to  their 
greatest  capacity,  their  protoplasm  appears  to  be  stimulated, 
perhaps  by  the  very  distension,  and  in  consequence  to  allow 
water  to  transude  through  its  substance.  This  mode  of 
response  to  stimulation  is  not  infrequent  in  vegetable 
tissues  ;  indeed,  it  appears  to  correspond  to  the  response  of 
stimulation  of  a  gland  cell  of  the  animal  body.  When  one 
of  the  nerves  supplying  the  parotid  salivary  gland  is  stimu- 
lated by  an  electric  current  the  gland  pours  out  its  secre- 
tion in  a  very  similar  way,  by  modifying  the  permeability 
of  the  cell  protoplasm,  so  that  the  hydrostatic  pressure 
existing  in  the  cell  is  able  to  force  the  water  through  the 
living  substance  with  greater  facility  than  it  could  before 
the  stimulus  was  appreciated.  By  thus  modifying  the 
turgor  of  the  cell,  the  protoplasm  relieves  itself  of  the 


88  VEGETABLE  PHYSIOLOGY 

over-distension,  and  we  get  an  intermittent  pumping  action 
set  up,  which  has  a  certain  rhythm.  By  it  large  quan- 
tities of  liquid  are  continually  being  forced  into  the  axial 
stele.  This  rhythm,  which  is  comparatively  rapid,  must 
not  be  confused  with  another  rhythm  which  is  much  more 
gradual,  and  which  constitutes  what  is  called  the  periodicity 
of  the  root-pressure. 

When  transpiration  is  not  taking  place,  the  water  accu- 
mulates in  the  vessels,  and  its  presence  can  then  very  readily 
be  demonstrated,  and  the  force  of  the  root-pressure  measured. 
If  a  vine  stem  is  cut  through  in  the  early  spring  before  its 
leaves  have  unfolded,  a  continuous  escape  of  water  takes 
place  from  the  cut  surface,  and  the  vine  is  said  to  bleed. 
The  phenomenon  is  not  peculiar  to  the  vine,  but  is  exhibited 
by  most  other  terrestrial  plants. 

In  plants  which  have  a  large  woody  system  the  accumu- 
lation of  water  in  the  vessels  can  only  be  demonstrated 
while  the  absence  of  leaves  renders  transpiration  impossible. 
Many  herbaceous  plants  show  a  similar  phenomenon  daily, 
owing  to  the  intermission  of  transpiration  during  the  night. 
In  these  cases  it  is  not  necessary  to  cut  the  axis  at  all ;  the 
accumulation  of  water  extends  to  the  whole  of  the  plant.  In 
the  early  morning  the  plants  show  a  certain  exudation  of 
water  from  the  tips  or  apices  of  the  leaves,  drops  accumulat- 
ing on  their  surfaces.  Alchemilla  and  Tropceolum  especially 
display  this  phenomenon,  which  is  due  to  the  over-tur- 
gescence  of  their  tissues,  brought  about  by  the  pumping 
action  of  their  roots. 

This  phenomenon  of  setting  up  a  hydrostatic  pressure 
causing  an  exudation  of  water  is  not  confined  to  roots. 
Whenever  the  active  living  cells  of  the  stem,  or  even  of 
the  leaves,  force  water  into  the  vessels,  the  same  exudation 
can  be  noticed.  It  can  be  shown  by  burying  the  cut  ends 
of  young  stems  of  grasses  in  wet  sand  ;  after  a  time  drops 
of  water  ooze  out  of  their  projecting  upper  ends.  If  the 
leafy  branches  of  some  trees  are  immersed  in  water  so  that 
only  the  cut  ends  project,  the  leaves  can  absorb  water  and 


ROOT-PBESSUBE 


89 


force  it  through  the  stem,  so  that  an  exudation  after  a  time 
can  be  noticed  to  take  place  from  the  cut  surface  which  is 
not  immersed.  A  similar  exudation  can  be  caused  to  take 
place  from  the  hypha3  of  fungi  and  from  the  tissues  of 
mosses. 

We  must,  however,  be  cautious  not  to  attribute  every 
escape  of  water  from  a  plant  to  this  cause.  When  a  tree 
trunk  is  wounded  or  cut  on  a  warm  sunny  day  in  winter, 
there  is  frequently  an  exudation  of  water  from  the  wound. 
This  is  generally  due  to  purely  physical  causes,  being 
brought  about  by  the  expansion  of 
the  air  which  is  contained  in  the 
vessels  of  the  wood.  It  can  be 
artificially  produced  at  any  time 
in  winter  by  warming  a  freshly 
cut  piece  of  wood  ;  and  its  cause  in 
this  case  can  be  seen  to  be  physical 
by  the  fact  that  as  the  wood  cools 
the  water  in  contact  with  the  cut 
surface  is  again  absorbed,  owing  to 
the  contraction  of  the  air,  which 
was  expanded  by  the  warming. 

To  measure  the  root-pressure 
in  a  plant  the  apparatus  shown  in 
tig.  62  may  be  used.  It  consists  of 
a  T-piece  of  glass  tubing  (R),  which 
is  fastened  by  indiarubber  rings 
(r)  to  the  top  of  a  cut  stem,  such 
as  that  of  Helianthus.  To  the  side 
arm  of  the  tube  a  manometer  (q), 

with  a  capillary  bore,  is  attached  by  a  tightly  fitting  cork 
(ft),  and  the  T-piece  is  filled  with  water  from  the  upper  end 
(&').  Mercury  is  poured  into  the  manometer  till  it  stands 
at  a  level  a  little  below  the  cork  k,  and  the  aperture  ~k'  is 
then  tightly  closed.  As  the  root  continues  to  take  up  water, 
it  forces  it  into  the  tube  E,  whence  it  overflows  into  the 
proximal  arm  of  the  manometer,  causing  the  mercury  in 


90  VEGETABLE  PHYSIOLOGY 

the  two  limbs  to  be  at  unequal  levels.  By  the  displacement 
of  the  mercury  the  force  of  the  root-pressure  can  be 
estimated.  A  variation  of  the  apparatus  can  be  used,  in 
which  the  manometer  is  replaced  by  a  glass  tube  bent  at 
right  angles.  The  water  will  be  forced  through  this,  and 
can  be  collected  in  a  suitable  receiver,  and  its  amount 
ascertained. 

In  performing  the  experiment  it  is  best  to  allow  the 
apparatus  to  stand  for  some  time  before  closing  the  tube 
at  7c',  as,  if  the  plant  is  taken  while  transpiration  is  proceed- 
ing, the  vessels  of  the  stem  will  contain  air  at  a  certain 
negative  pressure,  and  a  certain  amount  of  water  will  be 
sucked  back  until  the  vessels  are  full.  As  soon  as  this 
condition  is  reached,  the  pumping  action  of  the  roots  will 
become  evident,  and  the  root-pressure  will  make  itself 
obvious. 

The  root-pressure  of  various  plants  has  been  measured 
by  different  observers  ;  an  idea  of  its  amount  may  be 
gathered  from  the  fact  that  a  medium-sized  Fuchsia  in  a 
pot  has  been  found  able  to  send  a  column  of  water  up  a 
tube  of  the  same  diameter  as  the  stem  to  a  height  of  twenty- 
five  feet. 

The  activity  of  the  roots  will  depend  upon  various 
conditions,  of  which  temperature,  both  of  the  air  and  of 
the  soil,  is  one  of  the  most  important.  The  exudation  of 
water  has  been  observed  at  temperatures  as  low  as  freezing 
point,  but  most  plants  will  not  show  it  below  about  5°  C., 
and  as  the  air  becomes  warmer  the  quantity  of  water  given 
off  increases.  Warming  the  soil  of  the  pot  in  which  is  the 
plant  under  observation  also  increases  the  now. 

Of  other  influences  which  exert  an  effect  upon  the 
activity  of  the  roots  may  be  mentioned  oxygen.  Like  all 
other  vital  actions,  the  absorptive  power  of  the  root-hairs 
depends  upon  their  being  in  a  healthy  condition,  and  this 
cannot  be  maintained  in  any  protoplast  without  the  due 
performance  of  respiration.  The  character  of  the  soil  must 
also  be  considered.  Without  an  adequate  supply  of  moisture 


EOOT-PKESSUKE  91 

the  process,  of  course,  cannot  go  on,  and  a  disturbance  of 
the  normal  constituents  of  the  soil  will  lead  to  modifications 
of  the  process.  If  there  is  too  great  a  preponderance  of 
neutral  salts  such  as  sodium  chloride  or  potassium  nitrate, 
so  that  the  liquid  presented  to  the  roots  is  practically  a 
saline  solution,  the  exudation  will  cease  ;  indeed,  under 
such  circumstances  water  may  actually  be  withdrawn  from 
the  plant. 

Eoot -pressure  is  continually  at  work  while  the  trans- 
mission of  water  is  going  on  ;  but  it  is  not  easily  seen  later 
in  the  year  when  the  development  of  the  leaves  has  caused 
an  active  transpiration  to  proceed.  If  the  stem  of  the  vine 
be  cut  in  July  instead  of  in  March,  no  bleeding  follows  the 
wound.  This  is  not,  however,  due  to  the  absence  of 
activity  in  the  roots,  but  to  the  fact  that  the  copious 
evaporation  of  transpiration  prevents  the  necessary  accu- 
mulation of  water  in  the  cavities  of  the  woody  elements. 
In  the  experiment  in  the  early  spring  the  conditions  were 
different ;  there  were  no  expanded  leaves,  and  the  water 
absorbed  and  sent  upwards  by  the  root  consequently  re- 
mained in  the  vessels  of  the  stem,  escaping  at  once  when 
the  latter  was  cut.  In  July  the  vessels  have  been  emptied 
by  the  transpiration,  and  there  is  no  accumulation  of 
water  to  overflow.  The  apparatus  described  will  show, 
however,  if  the  experiment  with  it  is  continued  for  some 
time,  that  root-pressure  is  still  at  work,  even  though  tran- 
spiration is  vigorous  until  the  stem  is  severed. 

The  force  of  root -pressure  must  therefore  be  regarded 
always  as  a  factor  in  maintaining  the  transpiration  current. 
It  is  continually  forcing  water  into  the  vessels  of  the  axis, 
and  the  fact  that  transpiration  prevents  an  accumulation 
there  does  not  show  that  the  influence  of  root-pressure  is  done 
away  with  as  soon  as  it  ceases  to  be  easily  demonstrated. 

The  root-pressure,  though  always  considerable,  is  not 
the  same  at  all  times  of  the  day  and  night.  It  can  be 
measured  by  observing  the  output  of  water  in  the  second 
form  of  the  apparatus  described  above,  measurements 


92  VEGETABLE  PHYSIOLOGY 

being  taken  every  hour  ;  or  in  the  first  form  of  the  appa- 
ratus the  manometer  can  be  fitted  with  a  float  carrying 
a  pen,  which  can  be  made  to  trace  a  continuous  line  on  a 
slowly  rotating  recording  surface.  The  line  will  be  found 
to  describe  a  curve,  showing  points  of  activity  varying  from 
maximum  to  minimum.  The  general  features  of  the  curve 
will  be  the  same  for  all  plants,  but  all  do  not  give  the 
maximum  at  the  same  time  of  the  day.  In  the  case  of 
Cucurbita  Melopepo  the  minimum  point  occurs  in  the  early 
morning ;  the  curve  rises  slowly  during  the  forenoon, 
reaching  its  maximum  soon  after  midday.  From  this 
point  it  falls  ;  sometimes  a  second  smaller  rise  takes  place 
towards  evening,  and  then  it  sinks  continuously  all  night. 
The  time  of  the  occurrence  of  the  maximum  point  varies 
in  different  plants,  but  in  all  it  appears  to  be  during  the 
afternoon.  In  Prunus  Laurocerasus  it  is  much  later  than 
in  Cucurbita.  The  points  of  maximum  and  minimum 
activity  appear,  however,  to  be  about  twrelve  hours  apart, 
so  that  there  is  a  complete  diurnal  cycle. 

There  may  be  noticed  in  some  trees  also  a  variation 
which  suggests  a  yearly  periodicity.  The  power  of  exud- 
ing water  is  lost  for  a  time  during  the  winter,  the  loss 
being  noticeable  at  different  times  in  different  trees.  Vitis 
vinifera  does  not  show  any  exudation  usually  in  January  ; 
Acer  platanoides  is  passive  in  November  ;  many  plants  will 
not  bleed  at  all  during  the  winter. 

The  causes  of  these  variations  in  the  activity  of  the 
absorbing  mechanisms  of  the  roots  are  still  obscure.  The 
annual  periodicity,  when  it  exists,  appears  to  be  connected 
with  conditions  which  lead  to  the  discontinuance  of  growth 
during  winter.  The  trees  pass  in  fact  into  a  state  that  may 
be  compared  to  hibernation.  The  daily  periodicity  does  not 
appear  to  depend  upon  variations  in  the  surroundings  of 
the  plant,  but  to  be  due  to  some  cause  or  causes  inherent 
in  its  constitution.  It  has  been  suggested  that  it  has  been 
induced  in  plants  by  long-continued  variations  of  external 
conditions,  particularly  those  of  illumination,  involved  as 


TBANSPIKATION  93 

these  are  in  the  alternation  of  day  and  night.  This  alter- 
nation, affecting  successive  generations  of  plants  through 
an  enormous  length  of  time,  may  have  impressed  upon  the 
protoplasm  a  peculiar  rhythm  of  greater  and  less  general 
activity,  which  has  become  ultimately  automatic  and  inde- 
pendent of  the  immediate  surroundings.  Of  this  the  vary- 
ing action  of  the  roots  may  be  a  particular  expression. 

It  is  remarkable,  however,  that  very  young  plants  do 
not  exhibit  this  diurnal  variation,  but  they  gradually 
acquire  the  power  of  doing  so  as  they  develop,  subject  as 
they  are  under  normal  conditions  to  the  alternation  of  light 
and  darkness.  In  many  cases,  again,  the  diurnal  periodicity 
is  not  manifest  at  all. 

The  effect  of  the  periodic  alternation  of  light  and  dark- 
ness cannot  in  any  case  have  been  originally  appreciated  by 
the  roots,  as  they  are  implanted  in  the  soil  and  so  escape 
its  influence.  If  it  was  originally  due  to  such  variations, 
these  must  have  been  impressed  upon  the  general  orga- 
nisation of  the  plant. 

TRANSPIRATION. — The  modified  evaporation  by  which 
the  protoplasts  get  rid  of  water  and  enable  the  contents  of 
their  vacuoles  to  be  continually  renewed  takes  place  ulti- 
mately from  the  surfaces  of  all  the  succulent  parts  of 
plants,  and  to  a  less  extent  from  portions  of  the  exterior 
which  are  covered  by  a  layer  of  cork.  Like  the  activity 
of  the  absorbing  organs  of  the  root,  it  is  essentially  a  vital 
process,  and  is  regulated  by  the  protoplasm  of  the  cells 
which  take  part  in  it.  As  we  have  seen,  it  is  usually  spoken 
of  as  transpiration. 

It  is  easy  to  demonstrate  the  fact  of  its  continuous 
existence  during  daylight  by  enclosing  a  plant,  or  part  ol 
one,  in  a  dry  glass  vessel  which  can  be  closed  so  as  to  admit 
no  air.  Very  soon  the  surface  of  the  glass  becomes  covered 
by  a  fine  dew,  which  is  the  condensed  vapour  that  has 
escaped  from  the  plant.  The  same  thing  may  be  seen  when 
a  vigorous  plant  is  covered  over  by  a  bell-jar,  the  water 
condensing  copiously  upon  the  sides  of  the  latter, 


94 


'.'  flf'a  *-• 
VEGETABLE  PHYSIOLOGY 


A  more  elaborate  method  of  demonstrating  transpiration 
consists  in  placing  the  end  of  a  cut  branch  in  a  small  glass 
vessel,  preferably  a  U-tube,  filled  with  water,  as  shown  in 
fig.  63.  The  branch  passes  through  the  cork  of  the  vessel 
in  such  a  way  as  to  prevent  any  escape  or  evaporation  of 
water  at  that  point.  Communicating  with  the  other  arm 
of  the  U-tube  is  a  side  tube,  bent  at  right  angles,  which  dips 
into  the  water  through  a  perforated  cork.  This  tube  is 
also  filled  with  water.  As  transpiration  proceeds  the  water 


Fia.  63. — APPARATUS  TO  DEMONSTRATE  TRANSPIRATION  OF  A  BRANCH. 


is  gradually  drawn  from  the  horizontal  tube,  and  its  pro- 
gress can  be  noted  by  arranging  a  scale  behind  it.  The 
stem  or  branch  should  be  kept  with  its  cut  end  immersed 
in  water  for  several  hours  before  being  placed  in  the 
apparatus,  as  its  vessels  contain  air  at  a  negative  pressure 
when  it  is  cut,  owing  to  the  transpiration  which  has  been 
taking  place  from  it  before  its  separation  from  the  plant. 
The  existence  of  this  negative  pressure  will  lead  to  an  imme- 
diate absorption  of  water,  which  might  be  mistaken  for  an 
active  transpiration. 
The  evaporation  takes  place  to  a  certain  extent  through 


CALIFORNIA   COLLEGI 

<rf   PHARMACY 

TKANSPIBATION 


95 


all  the  epidermal  cells  of  the  transpiring  organ,  but  not  to 
a  very  great  one,  the  degree  of  the  development  of  the 
cuticle  having  considerable  influence  upon  its  amount.  It 
is  carried  out  much  more  freely  through  the  thin  walls  of 
the  cells  abutting  upon  the  intercellular  spaces,  which,  as 
we  have  seen,  communicate  with  the  external  air  by  means 
of  the  stomata  and  the  lenticels.  Very  little  watery  vapour 
is  given  off  by  the'Jatter,  so  that  by  far  the  greater  amount 
that  is  exhaled  passes  through  the  stomata.  Transpiration 
is  consequently  most  copious  from  the  leaves,  the  structure 


FIG.  64. — TRANSVERSE  SECTION  OF  THE  BLADE  or  A  LEAF,  SHOWING 
THE  INTERCELLULAR  SPACES  OF  THE  INTERIOR. 

of  the  lower  side  of  which,  in  dorsiventral  forms,  is  espe- 
cially favourable  to  it  (fig.  64).  If  a  leaf  is  taken  which  has 
stomata  upon  its  under  surface  only,  and  the  rates  of  watery 
exhalation  from  the  two  sides  are  compared,  it  will  be  found 
that  the  stomatal  gives  off  considerably  more  vapour  than 
the  other  surface. 

A  method  first  introduced  by  Stahl  enables  us  to  prove 
with  considerable  facility  that  the  escape  of  vapour  through 
the  stomata  is  much  greater  than  that  through  the  cuticular 
surface.  It  consists  in  applying  to  each  side  of  a  leaf  which 
has  stomata  only  on  the  under  surface,  a  piece  of  filter- 
paper  which  has  been  impregnated  with  a  solution  of 
cobalt  chloride  and  dried.  When  dry  this  paper  is  blue  in 
colour,  but  it  rapidly  becomes  pink  when  exposed  to  moisture. 
A  fresh  dry  leaf  is  taken  and  placed  between  two  pieces 


96 


VEGETABLE  PHYSIOLOGY 


of  the  cobalt-paper,  and  the  whole  put  between  two  dry 
sheets  of  glass  of  somewhat  larger  area.  In  a  very  short 
time,  often  in  less  than  a  minute,  the  paper  in  contact 
with  the  lower  side  of  the  leaf  becomes  pink,  while  the 
other  piece  remains  blue  for  a  considerable  time. 

The  amount  of  water  given  off  by  transpiration  varies 
in  different  plants.  In  the  sunflower  (Helianthus)  the 
amount  has  been  stated  to  be  T^  cubic  inch  of  water  per 
square  inch  of  surface  in  twelve  hours.  V.  Hohnel  has 
computed  that  a  birch-tree  with  about  200,000  leaves  may 

transpire  60  to  80  gallons  of 
water  during  a  very  hot  day. 
Doubtless,  however,  individual 
plants  show  a  considerable  variety 
in  the  amount.  This  copious 
evaporation  readily  explains  why 
the  bleeding  of  plants  from  wounds 
can  seldom  be  observed  when  the 
leaves  are  expanded  and  active. 

When  transpiration  is  exces- 
sive the  leaves  and  branches  lose 
their  turgescence,  become  flaccid, 
and  droop.  A  branch  which  has 
reached  this  condition  may  be 
revived  by  forcing  water  into  it, 
which  can  be  done  by  fastening 
it  into  one  arm  of  a  U-tube  con- 
taining water  (fig.  65),  and  pour- 
ing mercury  into  the  other.  The 

restoration  of  the  water  restores  the  turgescence    of    the 
tissues,  and  the  branch  regains  an  erect  position. 

The  exhalation  of  the  water  accumulated  by  root- 
pressure  in  the  closed  system  of  the  vessels  leads  to  a 
diminution  of  the  pressure  of  the  air  which  they  contain 
in  addition  to  the  water.  Indeed,  it  is  by  such  a  suction 
that  the '  air  is  originally  enabled  to  enter  the  vessels, 
being  drawn  into  them  from  the  intercellular  spaces. 


FIG.  65.— APPARATUS  TO  SHOW 
DEPENDENCE  OF  WITHERING 
UPON  Loss  OF  WATER. 


TKANSPIKATION  97 

Consequently,  while  transpiration  is  active,  there  is  a  nega- 
tive gaseous  pressure  existing  in  the  wood  vessels.  This 
continues  after  transpiration  ceases,  and  no  doubt,  like 
the  evaporation  itself,  it  is  of  assistance  in  maintaining  the 
upward  flow,  acting  as  it  does  in  the  same  direction  as  the 
pressure  in  the  turgid  cortex,  upon  which  it  exerts  a  con- 
siderable suction.  It  continues  until  the  entry  of  water 
from  the  root  causes  the  pressure  of  the  air  in  the  vessels 
to  be  equal  to  the  atmospheric  pressure.  This  negative 
pressure  is  of  considerable  importance  also  in  assisting  the 
movements  of  gases  in  the  plants. 

The  exhalation  of  watery  vapour  from  the  surface  of 
the  cells  is  not  a  process  of  simple  evaporation.  As  in  the 
other  phenomena  which  we  have  examined,  the  proto- 
plasm exercises  a  regulating  influence  upon  the  escape  of 
watery  vapour  from  the  cell.  If  the  amount  given  off 
from  a  measured  area  of  leaf-surface  is  compared  with 
the  quantity  evaporated  from  an  equal  area  of  free  water, 
the  latter  is  found  to  be  much  the  greater.  This  area  is 
certainly  much  less  than  the  area  of  the  cell- walls  actually 
involved,  which  abut  upon  the  intercellular  spaces  opening 
by  the  stomata  included  in  the  measured  area.  That  this 
difference  is  due  to  the  life  of  the  leaf,  and  consequently  to 
the  protoplasm,  is  seen  from  the  fact  that  a  dead  leaf  dries 
up  rapidly,  giving  off  its  water  more  quickly  than  a  surface 
of  freely  exposed  water.  The  cuticle  of  the  living  leaf  and  its 
cell-walls  are  consequently  not  the  causes  of  the  differences 
observed. 

The  ultimate  exhalation  of  watery  vapour,  we  have  seen, 
is  chiefly  carried  out  through  the  stomata  of  the  green 
parts,  at  any  rate  in  those  plants  which  possess  them. 
Each  stoma  is  situated  above  a  somewhat  conspicuous 
intercellular  space,  to  which  it  forms  an  outlet.  The 
stoma  originates  by  the  vertical  division  into  two  of  one 
of  the  cells  of  the  epidermis  which  is  usually  somewhat 
elaborately  differentiated  from  the  rest.  The  partition 
which  is  formed  between  the  two  daughter  cells  thickens 

7 


98 


VEGETABLE  PHYSIOLOGY 


slightly  and  splits  so  as  to  form  an  opening  between  them, 
which  does  not,  however,  extend  the  whole  length  of  the 


FIG.  66. SURFACE  VIEW  OP  PART  OF  THE  UNDER  SURFACE  OF  A  LEAF, 

SHOWING    THREE   STOMATA   IN   DIFFERENT    STAGES   OF    OPENING   AND 
CLOSING. 

wall,  so  that  the  two  cells  remain  attached  to  each  other  by 
their  ends  (fig.  66).    The  split  constitutes  the  stoma,  and  the 

two  cells  are  known 
as  the  guard  -  cells. 
They  are  commonly 
of  a  more  or  less 
semilunar  form  and 
contain  some  chloro- 
plastids,  a  point  in 
which  they  differ  from 
the  other  cells  of  the 
epidermis  in  the 
higher  plants.  Their  walls  become  thickened  and  cuticu- 
larised,  particularly  those  which  abut  upon  the  slit  and 
upon  the  intercellular  space  (fig.  67) ;  the  wall  which  is 


FIG.  67. — SECTION  OF  LOWER  EPIDERMIS   OF 
A  LEAF,  SHOWING  A  STOMA.     x  300. 


TKANSPIKATION 


in  contact  with  the  other  epidermal  cells,  however,  re- 
mains thin.  When  the  guard-cells  are  full  of  water, 
their  form  and  mode  of  attachment  cause  them  to  become 
curved  so  that  the  orifice  is  widely  open.  This  is  helped 
by  the  thickening  of  the  free  edges,  which  makes  it  difficult 
for  them  to  swell  in  the  direction  of  each  other.  When, 
on  the  other  hand,  they  lose  their  water,  they  relax, '  and 
their  edges  coming  into  contact,  the  aperture  between  them 
is  more  or  less  completely  closed  (fig.  66). 

The  edges  of  the  guard-cells  when  viewed  in  section  are 
slightly  convex  (fig.  67).  The  turgor  which  results  from 
the  imbibition  of  water  stretches  them  in  the  vertical  direc- 
tion as  well  as  in  the  horizontal  one.  This  tends  to  lessen 
the  vertical  convexity,  and  at  the  same  time  to  cause  a 
considerable  vertical  tension.  When  the  escape  of  water 
relieves  this  tension  the  thickened  upper  corners  of  the 
cell  recoil,  lessening  the  vertical  diameter  of  each  and 
increasing  this  convexity,  sometimes  bringing  the  con- 
vexities of  the  two  cells  into  contact  with  each  other,  and 
so  completely  closing  the  aperture. 

The  number  of  the  stomata  varies  very  considerably. 
The  following  table  will  give  some  idea  of  their  abundance 
in  leaves,  and  it  will  be  observed  that  the  number  of  stomata 
is  usually  greatest  in  those  leaves  from  whose  upper  surface 
they  are  entirely  absent. 


Stomata  in  one  Square  Inch  of  Surface 


Mezereon 
Pseony 
Vine   . 
Olive  . 
Holly 
Laurustinus 
Cherry-laurel 
Lilac  . 
Hydrangea 
Mistletoe 
Tradescantia 
House -leek 
Garden  Flag 
Aloe   . 
Yucca 
Clove  Pink 


Upper  Surface 

Lower  Surface 

none 

4,000 

none 

13,790 

none 

13,600 

none 

57,600 

none 

•  63,600 

none 

90,000 

none 

90,000 

none 

160,000 

none 

160,000 

200 

200 

2,000 

2,000 

10,710 

6,000 

11,500 

11,500 

25,000 

20,000 

40,000 

40,000 

38,500 

38,500 

7* 

100  VEGETABLE  PHYSIOLOGY 

The  modification  of  the  turgescence  of  the  guard-cells 
is  caused  by  the  transference  of  water  between  them  and 
the  other  cells  of  the  epidermis,  from  which  they  are  separated 
by  thin  walls.  The  vapour  which  is  in  the  intercellular 
space  below  them  does  not  penetrate  them,  the  walls 
abutting  on  the  space  being  thick  and  cuticularised.  The 
changes  in  the  turgor  of  the  guard-cells  are  effected  by 
their  own  protoplasm,  which  modifies  its  permeability  to 
water  in  consequence  of  their  receiving  a  stimulus  caused 
by  the  variations  in  the  quantity  of  water  in  the  plant  and 
especially  in  the  epidermis.  The  commencement  of  wilting, 
for  example,  causes  the  protoplasm  of  the  guard-cells  to 
allow  water  to  pass  from  them  with  increased  facility,  and 
they  consequently  diminish  the  size  of  the  opening,  checking 
the  loss  of  water  from  the  general  intercellular  space  system, 
and  hence  tending  to  restore  the  general  turgidity. 

It  was  held  till  recently  that  the  guard-cells  regulated 
their  turgor  by  producing  osmotic  substances  in  their 
interior  in  greater  or  smaller  amounts,  so  setting  up  osmotic 
currents  between  the  guard-cells  and  the  general  epidermis. 
This  was  supported  by  the  presence  of  chloroplasts  in  the 
guard-cells.  Quantitative  considerations  do  not  support 
this  view  of  the  mechanism.  A  short  period  of  darkness 
is  sufficient  to  close  the  stomata,  and  it  is  unlikely  that  so 
short  a  time  can  reduce  to  a  sufficient  extent  to  explain  the 
closure  the  amount  of  the  osmotic  substances  which  have 
been  drawing  water  into  the  guard-cells  till  the  failure  of 
the  light.  Nor  is  there  sufficient  evidence  that  darkness 
causes  a  diminution  of  the  quantity  of  osmotic  substance 
at  all. 

Transpiration  is  markedly  increased  by  sunshine,  rising 
to  many  times  its  original  amount  when  a  plant  is  trans- 
ported into  it  from  a  dim  light.  No  doubt  this  is  due  in  a 
very  large  measure  to  the  heat  rays  which  then  fall  upon 
the  plant,  and  which  would  raise  its  temperature  very 
dangerously  were  they  not  applied  to  the  evaporation  of 
the  water.  But  it  is  not  due  entirely  to  them,  nor  to  the 


TRANSPIRATION  101 

higher  temperature  of  the  air  accompanying  their  passage. 
The  light  has,  indeed,  an  influence  apart  from  the  heat. 
No  doubt,  so  far  as  the  visible  rays  of  the  spectrum  are 
converted  into  heat  vibrations  after  absorption,  they  must 
influence  transpiration  indirectly  in  this  way.  Besides 
acting  thus  indirectly,  light  has  a  direct  effect  upon  the 
process,  for  it  influences  the  size  of  the  stomatal  apertures. 
These  have  been  observed  to  be  open  during  the  day  and 
more  or  less  completely  closed  during  the  night.  The 
gaseous  interchanges  which  light  induces,  in  causing  the 
decomposition  of  carbon  dioxide  and  the  evolution  of 
oxygen,  on  the  whole  favour  the  exhalation  of  watery 
vapour.  When  green  plants  are  exposed  to  light  of  various 
colours  the  most  marked  increase  of  transpiration  is  caused 
by  the  light  of  which  the  plants  absorb  most.  This  can 
be  observed  not  only  in  the  green  parts  of  plants,  but  in 
those  which  are  not  green,  as  in  the  petals  of  the  flowers. 

The  fact  that  the  rays  which  are  absorbed  by  chloro- 
phyll are  the  most  active  in  promoting  the  process  has 
some  significance  when  it  is  remembered  that  the  guard- 
cells  of  the  stomata  contain  this  pigment.  The  nature  of 
the  action  of  chlorophyll  in  this  direction  is  not,  however, 
fully  understood. 

Apart  from  direct  radiation,  the  temperature  of  the 
air  and  its  hygrometric  condition  are  important  factors 
in  causing  an  increase  or  a  diminution  of  the  watery  vapour 
exhaled.  They  act  principally  by  exerting  an  influence 
directly  upon  the  evaporation  from  the  cells,  but  several 
indirect  effects  can  also  be  noticed.  The  general  movements 
of  water  in  the  plant,  as  well  as  its  absorption,  are  influenced 
particularly  by  variations  of  temperature,  and  the  latter  has 
also  an  effect  upon  the  width  of  the  stomatal  orifices.  A 
rise  of  the  external  temperature  causes  the  saturated  air  in 
the  intercellular  passages  to  expand,  as  the  air  acquires 
the  new  temperature  more  rapidly  than  do  the  tissues  of  the 
plant.  The  escape  of  vapour  is  consequently  accelerated 
as  the  temperature  rises,  even  though  the  rate  of  evaporation 


102  VEGETABLE  PHYSIOLOGY 

from  the  cells  into  the  intercellular  spaces  is  not  at  first 
affected. 

The  influence  of  the  hygrometric  condition  of  the  air, 
apart  from  changes  of  temperature,  can  be  seen  when  a 
plant  which  has  been  exposed  to  a  dry  atmosphere  till  its 
leaves  have  become  flaccid  is  transferred  to  one  saturated 
with  moisture.  After  a  short  time  the  drooping  leaves 
again  become  turgid.  This  is  not  due  to  an  absorption  of 
water  in  the  form  of  vapour  by  the  leaves,  but  to  a  diminished 
loss  by  the  checking  of  transpiration.  The  return  of  turgidity 
is  caused  by  the  accumulation  of  the  store  drawn  from  the 
earth  by  the  roots.  This  can  be  shown  by  comparing  the 
behaviour  of  two  plants  treated  in  the  way  described,  one 
of  which  is  allowed  to  remain  rooted  in  soil,  while  the  other 
is  taken  up  from  the  earth  and  exposed  in  that  condition  to 
the  saturated  air.  There  is  in  the  latter  case  no  recovery  of 
turgescence. 

The  temperature  of  the  soil  in  which  the  roots  of  a  plant 
are  embedded  has  also  an  influence  upon  the  exhalation 
of  watery  vapour,  which  increases  as  the  soil  is  warmed 
and  diminishes  as  it  becomes  cooler. 

If  the  protoplasts  of  the  cells  of  the  turgid  leaves  of  a 
branch  are  stimulated  by  violently  shaking  it,  the  leaves 
become  flaccid.  The  protoplasm  under  the  stimulus  allows 
more  water  to  pass  through  it  to  the  cell- walls,  and  hence 
evaporation  is  promoted.  The  effect  may  be  compared 
with  that  which  has  already  been  mentioned  as  set  up  in 
the  cells  of  the  cortex  of  the  root  by  their  over-distension 
by  the  water  which  accumulates  in  them  in  consequence  of 
the  continuous  osmotic  activity  of  the  root-hairs.  The 
stimulus  of  this  distension  is  responded  to  by  the  proto- 
plasm by  its  becoming  more  permeable  by  the  water  of  the 
vacuoles  of  the  cells.  The  response  made  by  the  protoplasts 
of  the  leaves  to  the  stimulus  of  shaking  may  help  to  explain 
the  flaccid  condition  observable  in  the  foliage  of  certain 
trees  after  the  prevalence  of  a  high  wind.  Besides  this 
effect  upon  the  protoplasm,  the  continuous  removal  of  the 


TKANSPIRATION 


103 


air  around  the  transpiring  organs  has,  no  doubt,  a  consider- 
able influence  upon  the  removal  of  the  watery  vapour  from 
their  intercellular  passages. 

The  effect  of  alteration  of  the  external  conditions  upon 
transpiration  may  be  investigated  by  means  of  Darwin's 
potometer,  which  enables  ap- 
proximately accurate  de- 
terminations of  its  amount 
to  be  made  from  time  to 
time.  This  instrument  is 
shown  in  fig.  68.  It  consists 
of  a  glass  tube  with  a  side 
arm  which  is  bent  upwards 
so  as  to  be  parallel  with  the 
tube  itself.  A  capillary  tube 
of  about  0'2  mm.  bore  is  fas- 
tened by  an  indiarubber  cork 
into  the  lower  opening  of  the 
tube  so  as  just  to  project 
beyond  the  cork.  A  con- 
venient length  of  the  capil- 
lary tube  is  about  20  cm. 
Its  lower  end  dips  into  a 
small  vessel  of  water,  ar- 
ranged so  as  to  be  easily 
withdrawn  from  the  tube. 
The  upper  orifice  of  the  poto- 
meter is  closed  by  a  tightly 
fitting  cork,  and  the  plant 
whose  transpiration  is  to  be 

observed  is  fitted  into  the  side  arm  by  means  of  an  india- 
rubber  band  or  tube  which  embraces  the  glass  arm  and  the 
end  of  the  cut  branch  so  as  to  make  a  water-tight  con- 
nection. The  whole  apparatus  must  be  filled  with  water,  and 
care  must  be  taken  that  no  escape  of  liquid  can  take  place 
at  any  of  the  junctions.  Any  air  that  finds  its  way  into 
the  instrument  during  the  arrangement  of  the  branch  in 


FIG.  68.— THE  POTOMJBTER. 


104  VEGETABLE  PHYSIOLOGY 

its  position  can  be  removed  by  causing  it  to  collect  at  the 
upper  portion  of  the  straight  tube  of  the  potometer.  To 
take  an  observation  of  the  rate  of  transpiration  of  the 
branch,  a  bubble  of  air  must  be  admitted  into  the  capil- 
lary tube  by  momentarily  removing  the  vessel  into  which 
it  dips,  and  replacing  it  as  soon  as  the  transpiration  has 
caused  the  air  to  enter.  The  bubble  of  air  must  be  of 
uniform  size  in  successive  readings,  to  ensure  that  the  latter 
shall  be  strictly  comparable  with  each  other.  The  bubble 
will  rise  in  the  tube,  and  finally  make  its  way  to  the  upper 
part  of  the  straight  limb  of  the  instrument,  the  rate 
at  which  it  travels  serving  as  an  index  of  the  rate  of  the 
transpiration.  The  capillary  tube  should  be  marked 
by  a  transverse  line  a  few  millimetres  from  its  lower  end, 
and  by  means  of  a  stop-watch  the  time  taken  by  the 
bubble  to  rise  from  this  mark  to  the  free  end  of  the  tube 
should  be  observed.  The  branch  may  be  covered  by  a 
bell-jar,  so  that  the  variations  of  temperature,  moisture, 
&c.,  of  the  air  surrounding  it  can  be  controlled  during  a 
series  of  observations.  Less  accurate  observations  can  be 
made  by  substituting  for  the  capillary  tube  a  tube  of  wider 
bore  bent  at  right  angles  a  little  below  the  orifice  of  the 
potometer,  and  affixing  to  it  a  scale  by  means  of  which 
the  rate  of  passage  of  the  column  of  water  in  the  tube  can 
be  observed  (fig.  63). 

According  to  the  variations  in  the  external  conditions  of 
the  plant,  including  all  the  features  already  alluded  to,  the 
amount  of  watery  vapour  transpired  is  continually  changing. 
The  most  favourable  conditions  being  afforded  in  summer, 
it  is  not  to  be  wondered  at  that  transpiration  attains  an 
annual  maximum  during  that  season.  It  does  not,  however, 
entirely  cease  during  the  winter,  though  it  is  reduced  to  a 
minimum,  especially  in  the  case  of  such  trees  as  shed  their 
leaves  in  the  autumn. 

Apart  from  such  changes  in  the  external  conditions, 
transpiration  appears  to  show  no  independent  periodicity, 
differing  in  this  respect  conspicuously  from  root-pressure. 


TBANSPIKATION 


105 


It  is,   however,   very   sensitive   to  slight   changes   in  the 
environment. 

It  was  mentioned  in  an  earlier  part  of  this  chapter  that 
the  force  of  transpiration  is 
of  considerable  assistance  in 
maintaining  the  upward  flow 
of  water  from  the  roots.  The 
apparatus  shown  in  fig.  69 
enables  this  to  be  demon- 
strated. The  cut  end  of  a 
branch  is  connected  by  an 
air-tight  joint  with  a  glass 
tube  filled  with  water,  the 
lower  end  of  which  dips  into 
a  vessel  of  mercury.  As  the 
water  is  transpired,  a  certain 
quantity  of  mercury  enters 
the  tube,  and  is  drawn  up  for 
some  considerable  distance 
by  the  suction. 

The  evaporation  from  the 
cells  takes  place,  as  we  have 
seen,  not  immediately  into 
the  external  air,  but  into  the 
intercellular  passages  of  the 
plant.  The  force  causing 
this  suction,  so  far  as  it  is 
due  to  evaporation,  is  there- 
fore localised  in  the  surface 
film  formed  in  the  evapo- 
rating cell- walls.  Such  an 
evaporation  has  been  shown 
by  Strasburger  to  be  capable 

of  raising  a  current  of  water  through  pieces  of  dead  wood 
which  have  been  soaked  and  injected  with  water. 

OSMOSIS   IN   THE  LEAVES. — There  is  reason  to  believe, 
.however,  that  a  third  factor  in  the  ascent  of  the  stream  is 


FIG.  69. — APPARATUS  TO  SHOW  THE 
SUCTION  CAUSED  BY  TRANSPIRA- 
TION. (After  Detmer.) 


106  VEGETABLE  PHYSIOLOGY 

interposed  between  the  forces  of  root-pressure  and  the 
evaporation  described.  The  water  is  passed  from  the  wood- 
vessels  or  conduits  to  the  evaporating  cells  through  a  varying 
thickness  of  parenchyma  (fig.  70),  which  is  kept  turgid 
during  active  transpiration.  The  turgid  condition  of  the 
cells  is  maintained  by  osmosis,  just  as  is  the  similar  condition 
in  the  roots.  The  vessels  abutting  on  the  parenchymatous 
cells  are  well  supplied  with  water,  which  is  in  their  cavities 
and  which  saturates  their  walls.  The  cells  contain  sub- 
stances of  an  acid  reaction,  and  exert  a  high  osmotic 
pressure.  We  cannot  doubt  that  an  osmotic  How  takes 


FIG.  70. — ENDING  OF  A  FIBRO-VASCULAR  BUNDLE  IN  THE 
PABENCHYMA  OF  A  LEAF. 

place  from  the  vessels  through  their  walls  into  the  paren- 
chyma of  the  leaf,  and  that  the  turgidity  of  the  tissue  of  the 
leaf  is  due  to  it  as  much  as  is  that  of  the  cortex  of  the  axis. 
Kesearches  carried  out  by  Dixon  show  that  this  osmotic 
force  plays  a  very  important  part  in  supplying  the  water  to 
the  evaporating  surfaces.  If  the  end  of  a  cut  branch  is 
immersed,  in  any  of  the  forms  of  apparatus  described,  in  a 
solution  of  a  salt  which  will  plasmolyse  these  cells  by  destroy- 
ing their  turgescence,  such  as  the  sodium  chloride  which  we 
have  already  seen  capable  of  doing  so,  the  rate  of  transpira- 
tion continues  without  much,  if  any,  diminution  till  the 
salt  can  be  detected  in  the  leaves,  when  it  suddenly  falls  off. 
This  takes  place  though  there  is  no  interruption  of  the 


TEANSPIKATION  107 

continuity  of  the  fluid  in  the  channels  of  the  transpiration 
current.  From  this  point  onward,  instead  of  evaporation 
sucking  up  water  from  the  root,  it  gradually  leads  to  a  drying 
of  the  leaf.  A  similar  result  is  brought  about  by  raising 
the  temperature  of  the  transpiring  branch  to  such  a  point 
as  will  kill  the  protoplasm  of  the  cells.  As  these  die  the 
evaporation  is  unchecked  at  first,  but  gradually  the  water 
is  taken  from  their  interior  and  no  more  is  supplied.  The 
cells  rapidly  become  flaccid,  the  leaves  droop,  and  the 
total  quantity  of  vapour  exhaled  is  materially  lessened,  the 
intercellular  passages  soon  becoming  partially  obstructed 
by  the  collapse  of  the  cells  abutting  upon  them.  The 
experiment  does  not  interfere  with  the  continuity  of  the 
water-stream,  but  as  soon  as  the  cells  are  made  unable  to 
retain  their  turgidity  by  the  interference  with  osmosis 
which  follows  the  death  of  the  protoplasm,  the  evaporation 
empties  the  cells  and  no  more  water  enters  them  to  replace 
what  has  been  lost.  As  we  have  seen  in  other  cases,  the 
death  of  the  protoplasm  is  followed  by  the  escape  of  the 
osmotic  substances,  which  do  not  leave  the  cells  during 
their  life.  The  mechanical  effects  which  follow  the  collapse 
of  the  tissue  are  the  consequence  of  the  assumption  of  a 
flaccid  condition,  and  they  intensify  the  check  to  the  escape 
of  watery  vapour  from  the  affected  organ. 

The  course  of  events  in  a  normal  leaf  during  active  tran- 
spiration appears  to  be,  then,  the  setting  up  of  a  tension 
in  the  parenchymatous  cells  of  the  leaf  by  evaporation 
from  their  surfaces,  which  tends  to  cause  them  to  collapse 
and  become  flaccid.  This  tendency  is  opposed  and  over- 
come by  a  greater  force  excited  by  the  turgescence  of  those 
cells  whose  osmotic  properties  exert  a  traction  upon  the 
water  in  the  conduits  or  wood-vessels.  Water  is  thus 
supplied  through  the  inner  walls  of  the  evaporating  cells  as 
quickly  as  it  is  lost  by  evaporation  from  the  surfaces  which 
abut  upon  the  intercellular  passages. 

Dixon  ascertained  that  the  osmotic  pressure  in  the 
leaves  of  transpiring  branches  of  the  Laburnum  amounted 


108  VEGETABLE  PHYSIOLOGY 

to  between  six  and  eight  atmospheres,  a  force  which  is 
capable  of  raising  a  column  of  water  to  a  height  of  more 
than  200  feet. 

Careful  consideration  of  the  facts  recorded  in  this  chapter 
shows  us  that  although  we  cannot  fully  explain  the  ascent 
of  the  transpiration  current,  we  can  see  that  it  ultimately 
depends  upon  the  behaviour  of  the  protoplasm.  All  the 
factors  which  aid  its  progress,  root-pressure,  transpiration, 
osmosis  in  the  cells  of  the  leaves,  are  largely  under  the 
control  of  the  living  substance,  and  are  particularly  influenced 
by  the  power  it  possesses  of  allowing  more  or  less  water  to 
pass  through  it,  according  to  its  condition.  Moreover  all 
the  external  influences  which  we  have  examined,  which  are 
brought  to  bear  upon  these  factors,  are  mainly  efficient  in 
as  far  as  they  affect  the  protoplasm  in  the  exercise  of  this 
power. 


109 


CHAPTEE  VII 

THE    AERATION    OF    PLANTS 

In  the  study  of  the  vital  processes  carried  on  by  the  proto- 
plast we  have  seen  so  far  how  entirely  it  is  dependent 
upon  the  free  access  of  water.  Another  factor  necessary 
for  its  existence  is  a  supply  of  air.  With  but  few  exceptions, 
and  those  occurring  among  the  lowliest  plants,  every  living 
organism  carries  out  a  series  of  gaseous  interchanges,  a 
feature  of  which  is  the  absorption  of  oxygen.  In  nearly  all 
cases  a  corresponding  amount  of  carbon  dioxide  is  ex- 
haled. In  most  plants — in  all,  indeed,  that  are  green — 
another  gaseous  interchange  takes  place,  carbon  dioxide 
being  absorbed  and  oxygen  simultaneously  eliminated. 
Every  protoplast  must  consequently  be  afforded  facilities 
for  carrying  out  gaseous  interchanges,  the  nature  and 
extent  of  which  vary  according  to  its  constitution.  The 
water  with  which  it  has  such  a  close  relationship  serves  as 
the  medium  through  which  such  interchanges  take  place, 
for  it  is  only  in  solution  that  gases  are  able  to  penetrate 
into  the  living  substance. 

In  the  case  of  those  protoplasts  which  live  in  a  watery 
environment,  the  latter  supplies  them  with  the  gases  they 
absorb  and  receives  those  which  they  exhale.  If  all  air  is 
withdrawn  from  the  water  in  which  they  are  living,  death 
speedily  ensues.  The  gases  enter  the  naked  protoplasts  by 
diffusion  through  the  film  of  water  which  is  in  contact  with 
their  free  surfaces.  In  the  case  of  those  which  have  a 
cell-wall  the  same  means  are  made  use  of.  Gases  in  solution 


110  VEGETABLE  PHYSIOLOGY 

can  diffuse  through  the  cell- wall,  which,  as  we  have  already 
seen,  is  saturated  with  water.  If  we  turn  to  those  unicellular 
or  filamentous  plants  which  live  on  the  surfaces  of  rocks 
or  tree-trunks,  the  process  is  only  slightly  modified,  for  the 
gases  of  the  atmosphere  readily  dissolve  in  the  water  which 
the  cell-walls  contain  and  diffuse  thence  into  the  interior  of 
the  cell. 

In  the  cases  of  those  more  bulky  plants  which  we  have 
especially  been  considering  in  the  last  chapter,  a  further 
mechanism  is  necessary,  as  the  external  air  cannot  gain 
access  into  the  interior  of  a  large  mass  of  cells  without 
special  arrangements  for  its  admission.  This  is  especially 
the  case  with  such  plants  as  are  possessed  of  protective 
mechanisms  like  the  corky  layers  of  the  bark,  or  the  strongly 
developed  cuticle  of  the  leaves.  The  arrangements  of 
the  structural  elements  in  these  plants  we  have  seen  to 
include  a  very  complete  system  of  intercellular  spaces, 
passages,  or  canals,  by  means  of  which  almost  all  the  con- 
stituent cells  are  placed  in  nearly  or  quite  complete  com- 
munication with  the  external  air.  The  intercellular  space 
system  has  consequently  a  very  important  function  to  dis- 
charge in  this  particular,  as  well  as  to  serve  as  the  means 
of  carrying  off  from  the  interior  the  aqueous  vapour  exhaled 
from  the  cells. 

The  intercellular  space  system  begins  to  appear  at  a 
very  early  period  in  the  development  of  the  young  plant. 
While  all  its  cells  are  merismatic,  as  is  the  case  when  it 
begins  to  emerge  from  the  seed,  they  are  united  completely 
together,  a  condition  which  persists  at  all  the  growing 
points  of  the  plant  as  its  age  increases.  During  the  young 
condition  the  aeration  of  the  internal  cells  is  provided  for 
by  the  slow  diffusion  of  the  gases  from  cell  to  cell,  absorp- 
tion from  the  exterior  by  the  external  cells  being  possible 
so  long  as  their  walls  are  not  cuticularised.  As  age  advances 
and  the  adult  condition  is  gradually  attained,  while  some 
of  the  cells  situated  deep  in  the  interior  are  dependent 
upon  a  similar  process,  the  majority  of  the  protoplasts  are 


THE  AEEATION  OF  PLANTS 


111 


FIG.  71.— CELLS  SPLITTING 
AT  THEIR  ANGLES  TO 
FORM  INTERCELLULAR 
SPACES. 


provided  with  access  to  the  air  by  the  formation  of  spaces 
due  to  the  splitting  of  certain  of  the  cell-walls,  and  the 
subsequent  partial  separation  of  the  cells.  Air  makes  its 
way  into  these  spaces  by  a  process  of  diffusion  outwards 
from  the  cells  abutting  upon  them, 
while  external  orifices  in  the  shape  of 
stomata  very  soon  make  their  appear- 
ance. The  various  constituents  of  the 
air  make  their  way  into  and  out  of 
each  cell  by  a  process  of  diffusion, 
being  dissolved  in  the  water  of  the 
cell-wall  or  escaping  from  such  a  moist 
membrane  according  to  the  conditions  existing,  and  the 
relation  between  the  internal  and  external  pressure  of  the 
particular  gas  in  question. 

As  soon  as  the  differentiation  of  the  tissue  in  the  growing 
part  of  an  organ  begins 
to  take  place,  the  for- 
mation of  the  inter- 
cellular spaces  can  be 
observed.  In  these 
regions  they  begin  *by 
a  splitting  of  the  wall 
between  two  contigu- 
ous cells  or  at  the 
angles  where  three 
cells  join  (fig.  71). 
The  crevice  soon  ex- 
tends and  may  make 
its  way  for  a  considerable  distance  round  any  particular 
cell.  The  cavities  so  come  into  communication  among  the 
cells,  each  of  the  latter  abutting  upon  a  single  one  or  upon 
several.  While  the  tissue  is  young  these  are  very  narrow 
and  slit-like,  or  are  only  visible  at  the  angles  when  the  cells 
are  polyhedral.  They  rapidly  become  larger  (fig.  72),  and 
in  some  parts,  particularly  in  the  interior  of  the  lower 
strata  of  the  mesophyll  of  dorsiventral  leaves,  they  may 


FIG.  72. — CORTEX  OF  ROOT,  SHOWING   INTER- 
CELLULAR PASSAGES  BETWEEN  THE  CELLS. 


VEGETABLE  PHYSIOLOGY 


oocupy  as  much  space  as  the  cells  themselves  (fig.  73). 
Light  appears  to  influence  their  development  somewhat, 


FIG.  73. — SECTION  OF  LEAF  SHOWING  THE  LARGE  INTERCELLULAR 
SPACES  OF  THE  MESOPHYLL. 

though  no  definite  relation  can  be  shown  to  exist  between 
the  degree  of  the  illumination  and  the  capacity  of  the 


FIG.  74. — SECTION  OF  LEAF  OF  Isoetes. 
a,  lacunar  cavities  ;   b,  vascular  bundle. 


cavities  formed.  Light  is,  however,  not  the  only  factor, 
and  probably  not  the  most  important  one,  in  determining 
their  extent,  for  they  are  usually  prominent  in  the  cortex 
of  roots,  which  receive  but  little  illumination.  The 


THE  AEKATION  OF  PLANTS  113 

explanation  of  the  relatively  large  development  in  this 
region  may  lie  in  the  fact  that  the  intercellular  cavities 
there  have  very  little  communication  with  the  outer  air,  as. 
stomata  do  not  exist  upon  roots.  There  is  thus  a  necessity 
for  a  larger  reservoir  of  air  than  in  parts  where  gaseous 
interchange  is  more  readily  effected. 
Besides  these  comparatively  narrow  channels  we  find 


— co.la. 


FIG.  75.— SECTION  OF  RHIZOME  OF  Marsilea. 
co.la.,  lacunae  in  cortex. 

cases  where  reservoirs  of  large  size  are  specially  developed. 
Such  structures  occur  in  the  leaves,  rhizomes,  and  roots 
of  aquatic  plants  which  are  nearly  or  entirely  submerged. 
Among  them  conspicuous  examples  are  afforded  by  the 
leaves  of  Salvinia  and  Isoetes  (fig.  74),  the  rhizome  of  Marsilea 
(fig.  75),  and  the  leaf  stalks  of  many  of  the  aquatic  Phanero- 
gams. These  are  developed  in  a  similar  manner  to  those 
already  described,  and  they  are  so  prominent  in  the  structure 

8 


114  VEGETABLE  PHYSIOLOGY 

that  a  section  shows  them  separated  from  each  other  by 
rows  of  cells  not  more  than  one  cell  thick  (tig.  76). 

In  some  cases  where  large  cavities  of  this  kind  occur 
the  mode  of  formation  is  different.  A  mass  of  tissue  lying 
in  the  position  of  the  subsequent  cavity  does  not  keep 
pace  in  its  development  with  the  growth  of  the  cells  sur- 
rounding it,  and  consequently  becomes  ruptured,  and  the 


FIG.*  76. — SECTION  OF  STEM  OF  Potamogeton,  SHOWING  AIR  PASSAGES 
IN  THE  CORTEX. 

cells  of  which  it  is  composed  are  gradually  destroyed,  leaving 
a  cavity  of  some  size.  Instances  of  this  mode  of  formation 
are  afforded  by  the  stems  of  Equisetum  (tig.  77),  the  haulms 
of  grasses,  and  the  hollow  stems  of  the  Umbelliferae  and  other 
plants. 

The  occurrence  of  these  large  air- containing  cavities  in 
partially  submerged  plants  may  be  explained  by  a  considera- 
tion of  their  habitat.  The  plant  is  in  contact  with  the  air 


THE  AEKATION  OF  PLANTS 


115 


by  only  a  very  small  portion  of  its  surface  ;  the  leaf-stalk 
of  Nymph  sea,  for  example,  is  always  submerged,  and  only 
the  floating  lamina  can  obtain  a  direct  supply  of  air.  The 
stomata  are  placed  upon  the  upper  surface,  and  afford  its 
only  means  of  entrance.  The  stems  and  roots  are  also  cut 
off  from  air  by  being  placed  either  in  water  or  in  mud.  The 


FIG.  77. — POETION  OF  AEKIAL  STEM  OF  A  SPECIES  OF  Equisetum. 
a,  cortical  lacuiia ;    b,  lacuna  iu  vascular  bundle ;    c,  chlorophyll-containing  cells. 

protoplasts  of  submerged  plants  are  almost  entirely  dependent 
upon  the  reservoir  of  air  which  the  body  of  the  plant  can 
contain,  a  small  quantity  only  entering  by  diffusion  from 
the  water  into  their  epidermal  cells. 

The  air  cavities  which  arise  in  the  stems  of  terrestrial 
plants,  such  as  the  grasses,  are  probably  not  primarily 
developed  with  a  view  to  the  aeration  of  the  plant,  but 
are  rather  intended  to  economise  the  material  used  in 


116 


VEGETABLE  PHYSIOLOGY 


construction.  The  hollow  stems  with  n  rigid  periphery, 
strengthened  at  intervals  by  diaphragms,  such  as  occur  at 
the  nodes  of  these  organs,  are  especially  adapted  to  main- 
tain an  upright  position  with  comparatively  little  expendi- 
ture of  material.  A  somewhat  similar  mechanism  is  met 


FIG.  78. — PORTION  OF  SECTION  OF  STEM  OF  HUSH,  SHOWING  STELLATE 
TISSUE  OF  THE  PITH,  WITH  LARGE  INTERCELLULAR  SPACES. 


with  in  the  stellate  parenchyma  of  the  stems  of  the  Bushes 
(fig.  78).  There  is  little  doubt,  however,  that  these  spaces 
are  of  great  assistance  in  promoting  the  aeration  of  the 
whole  structure. 

As  has  been  already  mentioned,  the  external  orifices  of 
the  system  of  the  intercellular  spaces  are  the  stomata  of 


THE  AEKATION  OF  PLANTS  117 

the  green  parts.  In  woody  and  corky  parts  these  are  supple- 
mented by  the  lenticels.  The  evidence  for  this  statement 
does  not  consist  only  of  microscopic  examination  of  the 
tissues.  A  direct  proof  can  he  afforded  by  a  simple  experi- 
ment. If  the  lamina  of  a 
leaf  is  immersed  in  water, 
air  can  be  driven  through 
it  by  subjecting  the  cut  end 
of  the  petiole  to  gaseous 
pressure  by  means  of  an 
air-pump,  or  even  by  the 

effort    Of    the    lungS    of    the          FIG.  79.— SECTION  OF  A  LENTICEL. 

observer,   and  can  be  seen  i,  lenticei ;  JM,  cork  layer, 

to  emerge  from  the  surface 

of  the  leaf  on  which  the  stomatal  apertures  are  situated. 
If  a  petiole  is  passed  into  a  glass  bottle  through  a  tightly 
fitting  cork,  and  covered  with  water,  while  the  lamina 
remains  in  the  air  outside  (fig.  80),  bubbles  of  gas  can  be 
made  to  emerge  from  its  cut  surface  in  a  continuous  stream 
by  reducing  the  pressure  above  the  water  by  means  of  an 
air-pump. 

The  facility  of  the  interchanges  will  largely  depend 
upon  the  number,  size,  and  position  of  these  orifices.  A 
lenticei  will  allow  more  gas  to  pass  between  its  loosely 
arranged  cells  than  will  a  stoma,  but  their  relative  numbers 
make  the  stomata  much  more  important  than  the  lenticels. 
In  most  cases  there  is  a  free  passage  through  the  stomatal 
pore,  but  in  others  considerable  difficulty  is  caused  by 
the  aperture  being  sunk  in  the  epidermis  or  situated  in  a 
depression  of  the  leaf.  In  the  rolled  leaves  of  heaths  and 
certain  grasses  this  difficulty  is  frequently  partially  com- 
pensated by  the  lacunar  character  of  the  parenchyma 
which  is  in  the  immediate  neighbourhood  of  the  stomata 

(fig-  81). 

It  must  be  noted  in  this  connection  that  the  stomata 
and  the  lenticels  are  passive  with  regard  to  the  process  of 
aeration,  and  do  not  exert  an  active  influence  upon  it. 


118 


VEGETABLE  PHYSIOLOGY 


The  variations  in  the  width  of  the  stomatal  apertures 
which  are  of  so  much  importance  in  the  regulation  of  tran- 
spiration must  be  regarded  as  hearing  upon  that  function 
alone,  being  caused  by  fluctuations  in  the  amount  of  water 
in  the  plant.  They  serve  automatically  to  preserve  the 
plant  from  excessive  loss  of  water,  but  they  have  no  direct 


FIG.  80. — APPARATUS  TO  SHOW  CONTINUITY  OF  INTERCELLULAR 
SPACES  IN  THE  LEAF.     (After  Detmer.) 


regulating  influence  upon  the  interchange  of  gases.  Indeed, 
when,  from  flaccidity  of  the  leaves  or  from  other  causes, 
they  close,  the  aeration  of  the  plant  is,  to  a  certain  extent, 
interfered  with,  if  not  suspended — a  consideration  which 
will  help  us  to  understand  why  a  plant  needs  to  contain  so 
large  a  reservoir  of  air  as  is  afforded  by  its  intercellular 
spaces.  The  volume  of  this  reservoir  varies  considerably 


THE  AEKATION  OF  PLANTS  119 

in  different  plants,  as  has  already  been  shown,  linger  has 
put  on  record  measurements  of  the  relative  volumes  of  air 
and  cellular  tissue  in  the  leaves  of  forty-one  species  of 
plants.  These  were  found  to  range  from  77  :  1000  in  Cam- 
phor a  officinalis,  where  it  was  least,  to  713  :  1000  in  Pistia 
texensis,  in  which  it  was  greatest. 

The  movements  of  the  air  in  the  intercellular  space 
systems  of  plants  depend  almost  entirely  upon  the  physical 
processes  of  diffusion.  The  entrance  and  exit  of  air  from 
the  exterior  are  generally  possible,  occasions  when  the 


Fio.  81.— TRANSVERSE  SECTION  OF  ROLLED  LEAF  OF  HEATH  WITH 
STOMATA,  st,  IN  THE  GROOVE. 


orifices  are  completely  occluded  being  very  rare.  It  does 
not,  however,  at  all  follow  that  the  atmosphere  in  the 
spaces  has  the  same  percentage  composition  as  the  external 
air.  When  we  consider  that  it  is  the  source  of  the  supply 
of  the  gases  used  in  the  metabolism  of  the  plant,  and  the 
recipient  of  those  which  are  from  various  causes  exhaled,  it 
becomes  evident  that  this  is  not  the  case.  Nor  is  its  com- 
position uniform  for  even  a  short  time,  as  the  various 
processes  which  subtract  from  or  add  to  it  take  place  in 
different  parts  with  very  different  rapidities.  At  the  same 
time  there  is  a  tendency  for  it  to  become  uniform  according 
to  the  laws  of  the  diffusion  of  gases. 


120  VEGETABLE  PHYSIOLOGY 

The  amount  of  nitrogen  varies  but  little.  This  gas 
has  a  certain  feeble  solubility  in  water,  and  a  small  quantity 
goes  into  solution  in  the  water  which  saturates  the  cell- 
walls  ;  but  as  such  nitrogen  is  not  made  use  of  in  the  cells, 
its  absorption  very  speedily  ceases,  the  cell-sap  not  being 
able  to  contain  more  than  a  trace  of  it.  The  percentage 
of  nitrogen  in  a  volume  of  gas  obtained  from  a  plant  may 
not  correspond  with  the  percentage  in  an  equal  volume  of 
air,  but  this  will  result  from  an  interference  with  the  amount 
of  oxygen  and  carbon  dioxide,  and  not  be  due  to  an  absorp- 
tion or  exhalation  of  nitrogen,  neither  of  which  takes  place 
to  an  appreciable  extent. 

The  variations  in  composition  which  are  noticeable  are 
due  to  two  processes  which  are  characteristic  of  the  vital 
processes  of  green  plants.  As  we  shall  see  in  a  subsequent 
chapter,  all  the  green  parts  of  plants  are  during  daylight 
engaged  in  absorbing  carbon  dioxide  from  the  air,  and 
exhaling  oxygen  into  it.  In  such  parts  this  interchange 
takes  place  with  considerable  energy,  and  the  composition 
of  the  air  in  their  intercellular  spaces  varies  accordingly, 
becoming  relatively  much  richer  in  oxygen  than  it  is  in  the 
deeper  parts  which  are  not  illuminated,  and  which  contain 
no  green  colouring  matter.  An  interchange  in  the  opposite 
direction  goes  on  continually  wherever  there  is  living 
protoplasm,  for  this  is  always  absorbing  oxygen  so  long 
as  it  lives,  while  a  good  deal  of  carbon  dioxide  is  simul- 
taneously exhaled.  This  process,  unlike  the  other  one,  is 
not  confined  to  any  particular  part  of  the  plant,  nor  is  it 
ever  in  abeyance.  Thus  the  plant  shows  a  continuous  and 
universal  production  of  carbon  dioxide,  and  a  partial  and 
local  consumption  of  this  gas.  At  the  same  time  it  exhibits 
a  constant  demand  for  oxygen  everywhere,  and  a  tem- 
porary production  of  it  in  places.  The  composition  of 
the  air  in  the  intercellular  spaces  must  therefore  vary 
from  time  to  time,  and  from  place  to  place,  according  to 
the  intensity  and  the  localisation  of  these  changes. 

The  process  of  diffusion,  which  is  one  of  the  phenomena 


THE  AEKATION  OF  PLANTS  121 

characteristic  of  gases,  leads  to  a  constant  occurrence  of 
gaseous  currents  in  plants.  These  currents  may  he  influ- 
enced hy  various  properties  of  the  gases  concerned,  and  hy 
other  factors,  both  internal  and  external.  The  rate  at  which 
carbon  dioxide  is  absorbed  by  the  cell-wall  is  very  different 
from  the  rate  of  absorption  of  oxygen.  If  an  atmosphere 
containing  a  good  deal  of  the  former  gas  is  in  contact  with 
wet  cell-walls,  the  result  of  the  active  absorption  will  be  to 
set  up  a  stronger  current  to  that  spot  than  would  be  the 
case  if  oxygen  replaced  it.  Any  cessation  in  the  absorption 
of  carbon  dioxide  by  the  green  cells  owing  to  diminution  of 
light  must  be  attended  by  a  certain  variation  in  the  gaseous 
stream.  The  ways  in  which  alterations  in  the  absorption  of 
oxygen  will  affect  the  currents  will  also  be  readily  apparent. 
During  bright  sunlight,  when  both  processes  are  proceed- 
ing in  the  same  and  in  different  parts  of  the  plant,  local 
positive  pressures  of  either  oxygen  or  carbon  dioxide  may 
occur,  and  it  is  evident  that  the  direction  of  the  gaseous 
current  will  vary  very  much  in  consequence. 

The  structure  of  the  plant  has  a  certain  influence  on 
the  composition  of  its  internal  atmosphere.  The  epidermis 
of  most  terrestrial  plants  is  strongly  cuticularised,  while 
there  is  but  little  cuticle  to  aquatics.  The  entry  of  gases 
into  the  latter  is  accordingly  easier  than  it  is  into  the  former, 
penetration  into  which  must  take  place  through  the  stomata. 
Moreover,  the  larger  reservoirs  in  the  interior  of  aquatics 
serve  to  equalise  the  composition  of  the  internal  atmosphere, 
and  to  cause  it  to  resemble  more  closely  that  of  ordinary  air. 

Such  plants  again  as  contain  no  green  colouring  matter — 
for  example,  the  bulkier  Fungi,  which  require  provision 
for  the  supply  of  air  to  their  interior — have  only  the  one 
metabolic  process  in  which  the  interchange  of  oxygen  and 
carbon  dioxide  is  involved,  the  former  being  absorbed,  and 
the  latter  exhaled.  To  a  corresponding  extent,  therefore, 
the  gaseous  currents  are  simplified,  though  even  in  these 
plants  the  direction  and  the  amount  are  never  constant  for 
long  together,  the  metabolism  continually  varying. 


122  VEGETABLE  PHYSIOLOGY 

In  another  important  respect  the  internal  air  of  plants 
differs  from  that  of  the  atmosphere.  It  is  always  charged 
with  aqueous  vapour,  frequently  even  to  the  saturation 
point,  as  we  have  seen  in  connection  with  the  process  of 
transpiration. 

The  external  conditions  to  which  a  plant  is  exposed 
have  a  considerable  influence  upon  the  gaseous  currents. 
The  effect  of  light  upon  a  green  plant  has  already  been 
alluded  to.  The  influence  which  it  exerts  is  an  indirect  one, 
affecting  the  consumption  of  carbon  dioxide  and  the  libera- 
tion of  oxygen.  Nearly  all  the  vital  processes  are  subject 
to  modification  by  changing  external  conditions.  Tran- 
spiration we  have  seen  to  be  very  largely  influenced  thereby, 
and  the  varying  amounts  of  watery  vapour  exhaled  intro- 
duce fluctuations  m.  the  amounts  of  the  purely  gaseous 
interchanges.  The  influence  which  the  variation  of  the 
quantity  of  water  in  the  plant  exercises  takes  the  form 
especially  of  modifying  the  width  of  the  stomatal  apertures, 
and  hence  of  interfering  with  the  entry  and  exit  of  gases 
into  and  from  the  leaves. 

Mechanical  disturbances  due  to  wind  are  of  some 
importance,  generally  increasing  the  gaseous  interchanges. 
Diminution  of  the  turgidity  of  the  tissues,  amounting 
sometimes  to  flaccidity,  interferes  at  times  to  a  serious 
extent,  the  intercellular  spaces  becoming  narrowed  by  the 
falling  together  of  the  cell-walls,  a  phenomenon  which  is 
noticeable  also  in  the  partial  or  complete  closure  of  the 
stomatal  orifices,  due  to  the  flaccidity  of  their  guard-cells. 

Variations  of  barometric  pressure  and  of  temperature 
also  influence  to  a  considerable  extent  the  process  of  diffu- 
sion within  the  plant,  as  well  as  the  interchange  between 
the  interior  and  the  external  air. 

The  movements  of  the  air  in  the  plant  are  subject  to 
disturbance  also  by  the  setting  up  of  the  negative  pressure 
in  the  cavities  of  the  vessels  of  the  wood  which  we  have 
seen  to  be  caused  by  active  transpiration.  This  negative 
pressure  can  be  demonstrated  with  considerable  ease  in  the 


THE  AEEATION  OF  PLANTS  123 

cases  of  woody  stems,  but  it  can  be  seen  also  in  plants  in 
which  the  development  of  wood  is  only  very  slight,  having 
been  observed  in  some  cases  in  the  elements  of  the  central 
cylinder  of  some  of  the  stouter  Mosses. 

To  demonstrate  the  existence  of  the  negative  pressure 
in  the  vessels  of  the  stem,  a  young  plant  should  be  removed 
from  the  soil  and  allowed  to  become  flaccid.  The  stem 
should  then  be  partially  immersed  in  mercury  and  cut 
across  below  the  surface  of  the  latter.  The  mercury  will 
immediately  rise  to  some  distance  in  the  vessels,  being 
drawn  up  by  the  suction  exerted  by  the  negative  pressure 
therein. 

An  actual  positive  pressure  can  under  certain  conditions 
be  observed  in  the  intercellular  air-reservoirs  of  particular 
plants.  This  can  be  shown  by  cutting  the  stems  of  sub- 
merged plants  such  as  Myriopliyllum,  when,  if  they  are 
brightly  illuminated,  bubbles  of  gas  may  be  seen  to  emerge 
from  the  cut  end.  This  positive  pressure  appears  to  be 
due  to  a  considerable  production  of  oxygen  by  the  green 
parts  of  the  plant  under  the  conditions  of  illumination,  as 
it  varies  with  the  intensity  of  the  latter,  and  ceases  entirely 
in  darkness. 

It  is  well  that  we  should  lay  some  stress  upon  the  nature 
of  the  relation  which  the  stomata  show  to  the  processes  of 
gaseous  interchange.  Though  they  are  the  chief  means  of 
the  entry  of  gases  into  and  their  exhalation  from  the  plant, 
it  is  misleading  to  speak  of  them  as  the  organs  of  such  gaseous 
interchange.  The  actual  processes  of  interchange  take 
place  between  the  protoplasts  and  the  air  of  the  intercellular 
reservoirs,  so  that  the  latter  are  the  special  organs  devoted 
to  such  functions.  The  stomata  and  the  lenticels  are  merely 
the  openings  by  which  the  air  of  these  internal  formations 
communicates  with  the  outer  atmosphere.  The  true 
gaseous  interchanges  which  subserve  the  life  of  the  proto- 
plasts, and  hence  of  the  plant,  take  place  not  at  the  stomatal 
orifices,  but  completely  throughout  the  interior  of  the 
substance  of  the  plant. 


124  VEGETABLE  PHYSIOLOGY 


CHAPTEK  VIII 

THE  FOOD  OF  PLANTS.   INTKODUCTORY 

A  good  deal  of  misconception  exists  as  to  the  nature  of  the 
food  of  plants.  The  character  of  their  environment,  and 
the  absence  in  most  cases  of  any  means  provided  in  their 
structure  for  the  taking  in  of  any  material  having  a  com- 
position at  all  approaching  that  of  living  substance,  have 
led  to  a  not  unnatural  idea  that  they  feed  upon  simple  in- 
organic compounds  of  comparatively  very  great  simplicity. 
This  idea  has  found  considerable  support  in  the  fact,  which 
is  easily  ascertained,  that  such  bodies  are  those  which  are 
absorbed  in  the  first  instance.  By  their  roots  when  they 
live  fastened  in  the  soil,  or  by  their  general  surface  when 
they  are  inhabitants  of  water,  comparatively  simple  inor- 
ganic salts  are  found  to  enter  them  with  the  water  which 
they  take  up.  By  their  green  parts,  and  especially  by 
their  leaves,  carbon  dioxide  is  absorbed,  either  from  air  or 
water,  according  to  their  habitat.  A  study  of  the  whole 
vegetable  kingdom,  however,  throws  considerable  doubt 
upon  the  theory  that  these  compounds  are,  in  the  strict 
sense,  to  be  called  their  food.  Fungal  and  phanerogamic 
parasites  can  make  no  use  of  such  bodies  as  carbon  dioxide, 
but  draw  elaborated  products  from  the  bodies  of  their 
hosts.  Similarly  those  fungi  which  are  saprophytic  can  only 
live  when  supplied  with  organic  compounds  of  some  com- 
plexity, which  they  derive  from  decaying  animal  or  vegetable 
matter.  We  have  no  reason  to  suppose  that  the  living  sub- 
stance of  these  non-chlorophyllaceous  plants  is  so  radically 


THE  FOOD  OF  PLANTS  125 

different  from  that  of  their  green  relations  that  it  has  a 
totally  distinct  mode  of  nutrition. 

In  the  flowering  plants  we  find  a  stage  of  their  life  in 
which  the  nutritive  processes  approximate  very  closely  to 
those  of  the  group  last  mentioned.  When  the  young  sporo- 
phyte  first  begins  its  independent  life — when,  that  is,  it 
exists  in  the  form  of  the  embryo  in  the  seed — its  living 
substance  has  no  power  to  utilise  the  simple  inorganic  com- 
pounds spoken  of.  Its  nutritive  pabulum  is  supplied  to  it 
in  the  shape  of  certain  complex  organic  substances  which 
have  been  stored  in  some  part  or  other  of  the  seed,  some- 
times even  in  its  own  tissues,  by  the  parent  plant  from 
which  it  springs.  When  the  tuber  of  a  potato  begins  to 
germinate,  the  shoots  which  it  puts  out  derive  their  food 
from  the  accumulated  store  of  nutritive  material  which  has 
been  laid  up  in  the  cells  of  its  interior.  Considerable  growth 
and  development  can  take  place  without  the  access  of  any 
of  the  inorganic  substances  which  the  parent  plant  was 
continually  absorbing.  Fleshy  roots,  corms,  bulbs,  and  all 
bodies  which  are  capable  of  renewed  life  after  a  period  of 
quiescence,  show  us  the  same  thing  ;  the  young  shoots 
emerging  from  any  of  them  are  not  fed  upon  simple  inorganic 
bodies,  but  upon  substances  of  considerable  complexity, 
which  they  derive  from  the  tissues  of  the  structures  from 
which  they  spring. 

In  adult  plants  of  the  most  considerable  complexity  we 
find  instances  of  the  same  thing,  though  in  these  cases  it 
is  generally  rather  more  difficult  to  determine  it ;  the 
living  substance  is  nourished  by  materials  which  have 
been  constructed  by  it  and  stored  at  various  places  in  its 
tissues  till  their  consumption  has  been  called  for. 

What,  then,  are  these  substances  which,  in  the  strict 
sense,  constitute  the  food  of  plants  ?  We  can  ascertain 
what  are  necessary  by  inquiring  what  are  the  materials 
which  are  deposited  in  the  seed  for  the  nutrition  of  the 
embryo  during  the  process  of  germination.  This  process 
is  the  most  favourable  for  the  elucidation  of  this  point, 


126  VEGETABLE  PHYSIOLOGY 

because,  in  its  early  stages  at  any  rate,  the  nutrition  of  the 
young  plant  is  not  complicated  by  any  absorption  from 
the  surrounding  medium,  such  as  sometimes  rapidly  super- 
venes on  the  emergence  of  a  shoot  from  a  tuber  or  a  fleshy 
root.  We  find  the  seed  contains  in  some  part  or  other  of 
its  substance,  sometimes  even  in  the  embryo  itself,  examples 
of  great  classes  of  food-stuffs  which  are  the  same  as  those 
on  which  animal  protoplasm  is  nourished,  and  whose  presence 
renders  seeds  such  valuable  material  for  animal  consump- 
tion. As  these  disappear  during  the  development  of  the 
young  plant,  which  thus  evidently  grows  at  their  expense, 
we  cannot  doubt  that  they  form  its  food,  and  that  vegetable 
protoplasm  is  essentially  identical  with  animal,  at  any  rate 
so  far  as  its  methods  of  nutrition  are  concerned.  Proteins, 
carbohydrates,  fats  or  oils,  together  often  with  certain 
other  bodies  which  are  less  widely  distributed,  are  the 
materials  which,  in  various  forms,  are  met  with. 

But  to  be  sure  that  these  complex  substances  are  the 
food  of  all  plants,  we  must  ascertain  whether  they  can  be 
found  in  the  cells  of  those  plants  and  parts  of  plants  which 
we  find  absorbing  the  simple  inorganic  materials  of  which 
we  have  spoken.  If  we  study  the  protoplasm  of  a  living, 
active,  vegetable  cell,  and  treat  it  with  appropriate  solvents, 
we  can  extract  representatives  of  them,  or  of  some  of  them, 
from  its  substance,  in  the  interior  of  which  they  are  held 
sometimes  in  solid  amorphous  form,  sometimes  in  fine 
suspension  or  in  actual  solution.  The  nutrition  of  the 
protoplasm  can  only  take  place  when  these  substances  are 
brought  into  the  most  intimate  relations  with  it ;  from 
them,  no  doubt,  in  ways  not  yet  discovered,  it  builds  itself 
up,  and  by  its  own  decomposition  it  reproduces  many  of 
them.  The  details,  however,  of  the  interchange  of  matter 
between  the  living  substance  and  its  food,  the  way  in 
which  the  latter  is  transformed  into  the  former,  are  points 
about  which  almost  everything  essential  remains  still  to  be 
discovered. 

But  while  we  recognise  that  the  ultimate  nutrition  of 


THE  POOD  OF  PLANTS  127 

protoplasm  is  dependent  upon  its  receiving  a  supply  of 
such  materials,  we  are  face  to  face  with  the  fact  that,  with 
a  few  exceptions,  the  consideration  of  which  may  be  deferred, 
they  are  not  furnished  at  all  from  the  environment  to  the 
ordinary  green  plant,  and  often  only  partially  so  to  the 
saprophytic  fungus,  though  they  are  freely  obtained  from 
their  host-plants  by  parasites.  On  the  contrary,  we  find 
the  ordinary  green  plant  taking  in  by  ordinary  physical 
processes  carbon  dioxide  from  the  air,  and  water  contain- 
ing a  variety  of  salts  from  the  soil.  The  saprophytic 
fungus  may,  and  frequently  does,  obtain  from  its  surround- 
ings certain  compounds  of  ammonia,  together  with  some 
carbohydrate  bodies,  such  as  sugar.  We  can  ascertain  that 
if  these  different  compounds  are  supplied  under  suitable 
conditions  to  the  groups  of  plants  mentioned,  the  latter 
can  flourish  and  develop.  While  we  have  the  strongest 
grounds  for  holding  that  the  protoplasm  is  essentially 
similar  in  all  these  cases,  we  see  marked  differences  between 
them  with  regard  to  the  materials  which  they  absorb. 
The  substances  supplied  to  the  green  plant  are  utterly 
unlike  what  we  have  seen  to  be  the  actual  food  ;  the  sapro- 
phytic fungus  can  make  use  of  the  compounds  of  ammonia, 
but  absorbs  carbohydrates  as  such,  while  the  parasite, 
whether  fungus  or  phanerogam,  obtains  the  materials 
which  we  see  are  directly  capable  of  feeding  it. 

If  we  say  that  the  food  of  these  various  groups  of  plants 
varies  in  the  degree  of  its  complexity,  we  must  carefully 
consider  in  what  sense  we  use  the  term/ood.  In  the  nutrition 
of  the  green  plant  there  are  clearly  two  very  different 
processes  combined,  which  should  be  kept  carefully  distinct. 
We  have  the  absorption  of  the  raw  material  of  food  rather 
than  of  food  in  the  true  sense,  and  we  have,  following  such 
absorption,  the  expenditure  of  a  considerable  amount  of 
energy  upon  these  food  materials,  with  the  result  that  they 
are  worked  up  into  the  complex  compounds  which  we  find 
protoplasm  can  assimilate.  These  are  such  as  we  see 
stored  away  in  the  substance  of  the  plant  for  the  nutrition 


128  VEGETABLE  PHYSIOLOGY 

of  vegetable  substance  and  the  development  of  embryo,  bud, 
or  growing  plant. 

In  the  case  of  the  green  plant  this  power  of  ^constructing 
food  extends  to  all  the  classes  of  foodstuffs  ;  in  that  of  the 
saprophytic  fungus  it  only  applies  to  the  proteins  and  the 
fats,  the  carbohydrates  needing  to  be  supplied  to  it  as  such, 
as  we  have  seen. 

The  difference  between  food  and  the  raw  materials  from 
which  it  is  constructed  can  be  made  clearer  by  inquiring 
whether  such  simple  inorganic  bodies  as  the  green  plant 
absorbs  are  capable  of  nourishing  protoplasm  when  freely 
supplied  to  it.  If  they  are  the  true  food,  plants  everywhere 
should  be  able  to  make  use  of  them.  But  if  we  consider 
only  one  of  them,  the  carbon  dioxide  of  the  air,  we  find  this 
is  not  the  case.  The  plants  which  are  not  green — that  is, 
which  contain  no  chloroplasts — can  do  nothing  with  this 
gas.  So  long  as  a  seed  is  in  the  early  stages  of  its  germina- 
tion, it  is  surrounded  by  carbon  dioxide,  which  is  given  off 
by  its  own  protoplasm.  But  it  can  make  no  use  of  it,  and 
if  the  store  of  nourishment  provided  for  it  in  the  endosperm 
or  cotyledons  is  cut  off,  it  inevitably  dies  of  starvation.  A 
saprophytic  fungus  in  like  manner  is  dependent  for  its  life 
upon  the  absorption  of  such  a  compound  as  sugar,  and 
carbon  dioxide  cannot  aid  at  all  in  its  nutrition. 

Another  fact  throws  a  certain  light  upon  the  relation  of 
carbon  dioxide  to  the  feeding  of  a  green  plant.  If  such  an 
individual,  in  good  health  and  endowed  with  ample  vigour, 
is  removed  from  light  to  darkness,  though  this  gas  be 
supplied  in  appropriate  quantity,  it  can  make  no  use  of 
it.  The  gas  is  evidently  useless  for  immediate  nutrition, 
and  its  ultimate  utility  is  dependent  upon  its  being  sub- 
mitted to  the  action  of  some  mechanism  in  the  plant  which 
is  called  into  play  under  particular  conditions,  of  which 
adequate  illumination  is  one. 

Similar  considerations  apply  to  other  constituents  of  the 
materials  from  which  the  true  food  of  the  living  substance 
is  elaborated.  They  are  absorbed  in  quantity,  but  they 


THE  FOOD  OF  PLANTS  129 

do  not  become  food  until  a  considerable  amount  of  work 
has  been  done  upon  them  by  the  plant  itself. 

It  thus  appears  that,  in  the  strict  sense,  the  ordinary 
green  plant  does  not  absorb  its  food  from  without.  It 
takes  in  various  raw  materials  from  which  it  manufactures 
its  food  in  particular  parts  of  its  own  tissues. 

In  connection  with  the  nutrition  of  plants  we  have  thus 
to  deal  with  the  absorption  of  the  crude  food  materials, 
and  to  study  the  changes  which  they  undergo  after  such 
absorption.  But  this  is  not  all ;  the  food  which  is  manu- 
factured from  them  is  not  merely  prepared  in  answer  to 
the  immediate  requirements  of  the  moment.  A  consider- 
able excess  is  usually  constructed,  and  the  surplus  quantity 
is  stored  in  various  parts  of  the  plant's  body  for  subsequent 
consumption. 

The  food  which  is  thus  laid  up  in  seeds,  tubers,  bulbs, 
&c.  is  not  deposited  there  in  exactly  the  condition  in  which 
the  living  substance  requires  it,  so  that  there  remains  for 
us  to  consider  the  processes  of  storage,  and  the  changes 
which  the  stored  materials  subsequently  undergo  for  the 
purpose  of  feeding  the  living  protoplasm. 

The  construction  of  food  from  the  materials  absorbed  is 
one  of  building  up  complex  bodies  from  simple  materials. 

The  utilisation  of  the  stored  surplus  is  comparable  with 
the  digestion  which  is  so  marked  a  feature  of  animal  alimenta- 
tion, and  is  one  of  breaking  down  of  complex  bodies  into 
simpler  ones. 

The  actual  nutrition  of  the  protoplasm  shows  again  two 
distinct  phases  :  the  incorporation  into  its  substance  of  the 
ultimate  constituents  of  the  food,  or  its  assimilation,  is  a 
constructive  process  ;  it  is  in  turn  associated  with  a  destruc- 
tive one,  an  auto -decomposition  of  the  protoplasm  itself,  by 
which  simpler  bodies  are  produced  from  it. 

The  whole  round  of  changes  which  embraces  all  these 
operations  is  called  metabolism,  the  constructive  processes 
being  grouped  together  under  the  name  of  andbolism,  the 
destructive  ones  under  that  of  katabolism. 

9 


130  VEGETABLE  PHYSIOLOGY 

The  absence  of  well- differentiated  organs  set  apart  for 
the  discharge  of  these  separate  functions  makes  it  rather 
difficult  at  first  to  appreciate  their  independence.     In  most 
animal  organisms  such  a  differentiation  is  easily  seen,  but 
in  plants  the  cellular  structure  is  so  prominent,  and  the 
life  of  the  protoplasm  is  so  closely  related  to  its  condition 
in  the  cell,  that  attention  needs  to  be  specially  directed  to 
the  point.    Each  protoplast  is  dependent  upon  the  contents 
of  its  own  vacuole,  and  the  early  constructive  processes  in 
the  metabolism,  including  the  manufacture  of  food  in  such 
cells  as  carry  out  this  process,  may  take  place  in  it  side  by 
side  with  the  digestive  changes  and  at  almost  the  same 
time.     True,  a  certain  division  of  labour  can  be  noted,  but 
it  is  not  very  clearly  associated  with  particular  organs. 
The  leaf,  for  instance,  is  especially  concerned  in  the  manu- 
facture of  food,  but  it  is  mainly  so  by  virtue  of  the  chloro- 
plasts  which  its  cells  contain.     These  processes  can  go  on 
perfectly  well  in  other  parts  than  leaves  ;   indeed,  wherever 
there  are  chloroplasts  we  know  they  do.     Thus,  though  we 
associate  the  leaf  with  this  manufacture,  it  would  be  wrong 
to  speak  of  it  as  the  organ  to  which  this  process  must  be 
referred.    We   can   say   with    greater   accuracy    that   the 
chloroplast  is  the  organ  which  conducts  these  preliminary 
constructive  processes,  and  that  they  take  place  wherever 
the  chloroplasts  are  found.     The  wide  distribution  of  the 
latter,  however,  shows  us  that  there  is  no  specially  differen- 
tiated member  of  the  plant  set  apart  to  be  an  organ  for  this 
function.     In  the  same  way  the  digestive  process,  or  the 
utilisation  of  stored  products,  goes  on  wherever  there  are 
reservoirs  of  such  bodies,  and  takes  place  in  the  cells  of 
which  such  reservoirs  consist.     There,  and  there  only  for 
the  most  part,  unorganised  ferments  or  enzymes  are  found, 
instead  of  being  located  in  particular  glands,  as  in  many 
cases  in  the  animal  body.     These  reservoirs,  as  we  have 
already  seen,  and  shall  see  again  later,  are  found  in  the 
most  varied  regions  of  the  plant's  substance,  regions  more- 
over  which   differ   considerably   in   situation   in   different 


THE  FOOD  OF  PLANTS  131 

plants.    We   cannot,  therefore,   speak  of  a   differentiated 
organ  of  digestion. 

Starting,  then,  with  the  intricacy  of  the  metabolic  pro- 
cesses placed  before  us,  and  with  their  relations  to  each 
other,  we  may  begin  the  consideration  of  them  in  detail 
with  an  inquiry  into  the  preliminary  absorption  of  the 
materials  from  which  the  food  is  ultimately  made.  Even 
here  we  meet  with  some  complexity,  as  the  ordinary  green 
plant  shows  marked  differences  in  behaviour  from  its 
parasitic  relative  and  from  the  great  class  of  Fungi,  which 
possess  no  chlorophyll.  We  have  already  pointed  out  that 
the  construction  of  food  does  not  follow  exactly  the  same 
course  in  green  plants  and  saprophytic  fungi,  the  chief 
point  of  difference  being  seen  in  connection  with  the  carbo- 
hydrates. It  will  be  best  to  consider  first  the  ordinary 
terrestrial  green  plant,  noticing  in  passing  differences  in 
behaviour  shown  by  aquatic  and  epiphytic  forms. 


132  VEGETABLE  PHYSIOLOGY 


CHAPTER  IX 

THE  ABSORPTION  OF  FOOD  MATERIALS  BY  A  GREEN  PLANT 

We  have  seen  that  the  materials  which  protoplasm  is 
eventually  able  to  assimilate  or  incorporate  into  its  own 
substance,  and  which,  therefore,  constitute  its  food,  are  of 
a  similar  nature  to  those  deposited  in  seeds  and  other  store- 
houses of  nutriment.  We  know  further  that  these  are  not 
the  materials  which  an  ordinary  green  plant  absorbs  from 
the  environment  in  which  it  lives.  We  know  also  that  its 
structure  prevents  its  taking  in  anything  in  a  solid  form, 
and  that,  therefore,  everything  entering  it  must  either  be  in 
solution  in  the  water  which  it  is  almost  constantly  absorb- 
ing through  its  roots,  or  must  become  dissolved  in  the 
liquid  which  permeates  the  walls  of  the  cells  which  line  the 
intercellular  passages.  The  only  substances  that  can  be 
taken  up  under  these  conditions  are  certain  gaseous  con- 
stituents of  the  air,  and  various  inorganic  salts  which  are 
present  in  the  soil.  Between  such  raw  materials  and  the 
complex  products  which  are  needful  for  the  nutrition  of  its 
substance  there  is  a  great  difference,  and  the  manufacture 
of  the  latter  from  the  crude  materials  absorbed  constitutes 
a  very  important  part  of  the  metabolic  processes. 

There  are  several  ways  in  which  we  may  proceed  to 
discover  what  a  green  plant  absorbs  from  the  soil,  two  of 
which  especially  have  been  made  use  of  by  various  observers. 
The  first  is  known  as  the  method  of  water-culture.  It 
consists  in  cultivating  plants  with  their  roots  inserted  in 
water  containing  various  salts  in  solution,  and  observing 


ABSORPTION  OF  FOOD  MATERIALS  133 

what  effect  upon  their  growth  and  development  is  produced 
by  the  addition  of  certain  compounds  to  the  culture  fluid,  or 
how  the  absence  of  any  particular  salt  affects  their  well-being. 

In  carrying  out  experiments  in  this  way,  it  is  usual  to 
sow  some  large  seeds,  such  as  those  of  the  broad  bean,  in 
damp  sawdust,  and  allow  them  to  germinate.  When  the 
radicle  of  the  seedling  has  elongated  to  the  extent  of  about 
an  inch,  the  seed  is  placed  upon  a  perforated  cork  inserted 
into  the  neck  of  a  bottle  containing  the  liquid  which  is  the 
subject  of  the  investigation.  It  is  so  arranged  that  the 
radicle  dips  down  through  the  cork  into  the  liquid.  As 
growth  proceeds  the  radicle  develops  a  root-system  in  the 
way  appropriate  to  the  particular  plant  used,  which  absorbs 
from  the  liquid  the  salts  which  are  required  by  it,  so  far  as 
these  are  present.  At  the  same  time  the  plumule  grows 
upwards,  and  soon  a  shoot  appears,  which  develops  pari 
passu  with  the  root. 

By  this  method  various  plants  can  be  cultivated  with 
different  degrees  of  success  ;  in  some  cases  not  only  leaves, 
but  flowers  and  even  fruit  can  be  produced.  The  progress 
of  the  plant,  and  the  readiness  with  which  it  will  develop, 
will  depend  upon  the  salts  which  are  supplied  to  it  in  the 
water,  if  it  is  maintained  in  normal  conditions  of  light, 
temperature,  and  aeration.  In  preparing  the  solution, 
particular  mixtures  can  be  employed,  and  the  most  favour- 
able one  ascertained,  while  subsequent  analysis  of  the 
liquid  will  show  to  what  extent  the  various  constituents  of 
the  culture  fluid  have  been  abstracted  from  it. 

This  method  is,  however,  only  of  use  in  determining 
particular  points,  such  as  the  effect  of  the  presence  of  certain 
metals  in  particular  combinations,  or  the  influence  of 
different  concentrations  of  particular  substances.  It  does 
not  give  an  account  of  what  is  happening  to  a  plant  with 
its  roots  embedded  in  the  soil,  for  the  composition  of  the 
latter  cannot  be  compared  with  that  of  a  solution  definitely 
made  up  for  purposes  of  experiment.  The  composition  of 
the  soil,  as  we  have  seen,  is  very  far  from  uniform,  and  the 


134  VEGETABLE  PHYSIOLOGY 

constituents  which  are  within  the  reach  of  the  roots  of  two 
plants  growing  almost  side  by  side  may  naturally  be 
materially  different  in  their  proportions.  This  considera- 
tion makes  it  almost  or  quite  impossible  to  ascertain,  by 
observation  of  the  soil  and  the  plant  growing  in  it,  what 
are  the  substances  which  are  entering  its  roots. 

The  other  method,  which  is  of  much  more  general  applica- 
tion, consists  in  making  an  analysis  of  the  whole  body  of 
the  plant  after  its  removal  from  the  soil,  and  so  ascertaining 
what  chemical  elements  it  contains.  A  plant  gives  off  no 
solid  excreta,  and  consequently  whatever  it  absorbs  remains 
in  its  substance.  The  ultimate  composition  of  the  true 
nutritive  matters,  proteins,  carbohydrates,  fats,  &c.,  is 
known.  Such  an  analysis  having  shown  what  elements 
enter  into  the  composition  of  a  plant,  and  of  the  food  which 
it  has  stored  in  its  tissues,  it  becomes  possible  to  inquire 
into  the  manner  in  which  each  is  supplied  to  the  plant  under 
examination,  and  into  the  work  which  is  done  upon  them 
in  its  cells. 

As  previously  noticed,  the  structure  of  the  plant  demands 
that  all  the  materials  of  a  solid  character  shall  be  in  such 
a  solution  that  they  can  enter  its  substance  by  means  of 
the  processes  already  described  as  taking  place  through  the 
cell-wall.  Similar  considerations  apply  to  gases,  of  which 
there  is  considerable  absorption  by  all  plants,  whatever 
may  be  the  nature  of  their  habitat. 

The  details  of  absorption  vary  to  some  extent,  however, 
according  to  the  environment  of  the  plant.  Aquatic  plants 
can  absorb  water,  and  whatever  is  dissolved  in  it,  whether 
of  gaseous  or  solid  character,  by  all  parts  of  their  surface. 
Those  which  grow  with  their  roots  embedded  in  soil,  and 
their  shoots  exposed  to  the  air,  show  a  certain  division  of 
labour  in  this  respect.  The  mineral  constituents  obtained 
from  the  soil  are  taken  in  by  the  root-hairs  with  the  stream 
of  water  ;  those  of  a  gaseous  nature  mainly  find  entry 
through  the  leaves  and  other  green  parts. 

To  make  a  destructive  analysis  of  the  plant,  it  must  be 


ABSOKPTION  OF  FOOD  MATEKIALS  135 

dried  at  110°-120°  C.  to  drive  off  the  water  it  contains,  and 
it  must  then  be  carefully  burnt,  and  the  residue  of  the 
combustion  collected.  The  volatile  products  given  off  can 
also  be  absorbed  by  appropriate  methods,  and  their  nature 
and  amount  ascertained.  The  incombustible  residue,  which 
is  known  as  the  ash,  is  composed  of  several  metals  and  some 
other  elements,  which  vary  in  nature  and  amount  in  different 
cases.  An  analysis  of  this  ash  will  reveal  the  nature  of  its 
constituents,  but  it  will  not  tell  us  in  what  condition  or  com- 
bination they  existed  in  the  living  plant,  on  account  of  the 
various  chemical  changes  which  go  on  during  the  combustion. 

If  we  examine  the  food-stuffs  described  as  being  essential, 
we  find  that  proteins  contain  carbon,  hydrogen,  oxygen, 
nitrogen,  sulphur,  and  perhaps  pJiosphorus.  Carbohydrates 
and  fats  contain  only  the  first  three  of  these  elements. 

The  ash  of  plants  when  analysed  is  always  found  to 
contain  the  four  metals,  potassium,  magnesium,  calcium, 
and  iron.  These  are  not  present  in  the  metallic  condition, 
but  are  in  combination  with  various  acids,  forming  nitrates, 
sulphates,  chlorides,  carbonates,  phosphates,  &c. 

The  presence  of  these  nitrates,  sulphates,  &c.  must  not 
lead  us  to  infer  that  they  have  all  been  absorbed  as  such 
from  the  soil  and  retained  unaltered  in  the  plant.  Part,  no 
doubt,  may  be  accounted  for  in  this  way,  but  much  of  the 
nitrogen,  sulphur,  and  phosphorus  which  formed  part  of 
the  substance  of  the  plant  enters  into  combination  with  the 
different  metals  and  with  oxygen  during  the  combustion. 
Some  of  the  carbon  of  the  carbonates  found  may  have  had 
a  similar  origin. 

Besides  the  four  metals  mentioned,  various  plants  may 
individually  contain  larger  or  smaller  quantities  of  many 
other  elements  variously  combined.  We  find  sodium  very 
generally  present ;  less  frequently  so,  aluminium,  copper,  zinc, 
manganese,  silicon,  bromine,  and  iodine  ;  others  occur  only 
exceptionally  and  in  small  traces.  All  of  these  are  derived 
from  compounds  present  in  the  soil,  or  the  water  with 
which  they  are  in  contact  ;  indeed,  the  composition 


136  VEGETABLE  PHYSIOLOGY 

of  the  soil  in  which  a  plant  grows  determines  to  a  very  great 
extent  what  minerals  enter  it.  If  a  particular  substance  is 
soluble  in  the  liquid  which  the  root-hairs  absorb,  and  is  cap- 
able of  osmosis  through  their  membrane,  a  certain  quantity 
will,  by  ordinary  physical  processes,  be  taken  up  by  them. 

It  does  not,  however,  follow  that,  if  the  conditions  alluded 
to  are  realised,  absorption  of  a  particular  salt  will  go  on 
indefinitely.  The  quantity  of  any  substance  which  a  plant 
will  absorb  will  depend  upon  whether  it  is  made  use  of  in 
any  way,  or  can  be  deposited  in  its  tissues  in  an  insoluble 
form.  This  can  be  seen  most  easily  by  studying  the  be- 
haviour of  a  single  cell.  If  any  substance  which  enters  the 
cell  by  osmosis  is  used  in  its  metabolism,  it  will  be  quickly 
removed  from  the  sap  in  its  vacuole,  and  more  will  enter. 
If  not,  the  cell-sap  will  soon  have  taken  up  as  much  of  it 
as  it  can  contain,  and  the  absorption  of  that  particular 
substance  will  cease.  This  is  equally  true  of  such  a  com- 
plex of  cells  as  constitutes  a  plant,  though  the  time  of  the 
absorption  will  be  more  prolonged.  As  soon  as  all  the 
cells  of  the  complex  attain  a  condition  of  equilibrium  with 
regard  to  the  particular  salt  in  question,  no  more  will  be 
taken  up.  This  follows  from  the  nature  of  the  process  of 
osmosis.  If  the  substance  under  examination  is  withdrawn 
from  the  sap  in  any  part  of  the  plant,  and  made  use  of  for 
any  purpose,  or  deposited  in  the  cells  in  an  insoluble  form, 
the  condition  of  equilibrium  will  not  be  attained  so  long  as 
such  a  withdrawal  at  any  point  takes  place,  and  a  stream 
of  the  substance  will  flow  continuously  to  the  point  in  ques- 
tion, so  that  the  process  of  absorption  will  be  continuous  also. 

Some  of  the  materials  found  in  the  soil  are  readily 
soluble  in  the  water  which  surrounds  its  particles.  We 
have  already  seen  that  it  is  only  this  hygroscopic  water 
which  finds  its  way  into  the  root-hairs.  Such  salts  dis- 
solve in  this  water  and  can  enter  the  plant  without  diffi- 
culty if  they  are  capable  of  passing  through  the  protoplasm 
of  the  root-hair.  The  solution  of  the  salts  is  always  very 
dilute,  and,  on  account  of  the  ready  diffusion  that  takes 


ABSORPTION  OF  FOOD  MATERIALS         187 

place,  their  concentration  is  approximately  uniform  in  any 
particular  soil.  Other  salts  are  insoluble  in  pure  water, 
and  their  absorption  presents  more  difficulty.  Many  are 
soluble  in  water  which  contains  carbon  dioxide,  and  as 
considerable  quantities  of  this  gas  are  continually  being 
generated  in  the  soil,  the  water  there  is  charged  with  it, 
and  bodies,  otherwise  intractable,  are  thereby  brought  into 
solution  and  absorbed. 

The  power  of  water  containing  carbon  dioxide  to  effect 
the  absorption  of  such  substances  is  capable  of  easy  demon 
stration.  One  of  these  salts  is  calcium  sulphate  or  gypsum. 
If  a  plate  of  this  substance  is  placed  at  the  bottom  of  a 
flower-pot  and  the  pot  then  filled  with  moist  earth,  a  plant 
caused  to  grow  in  it  till  its  root  system  is  well  developed  will 
have  some  of  its  roots  closely  adpressed  to  the  gypsum 
plate.  After  a  time,  examination  will  show  the  surface  of 
the  plate  eaten  away  at  all  points  except  where  the  roots 
have  become  adpressed  to  it,  and  the  regions  covered  by 
the  latter  will  stand  out  in  slight  relief.  The  whole  sur- 
face will  have  been  subjected  to  the  action  of  the  water 
and  the  carbon  dioxide  it  contains,  except  where  it  has 
been  covered  by  the  roots,  and  the  solvent  action  will 
consequently  be  recorded. 

A  third  factor  which  must  be  considered  in  the  process 
of  absorption  is  the  acid  sap  which  the  root-hairs  contain. 
Not  only  does  the  acid  cause  water  to  enter  the  hair 
osmotically,  but  a  little  of  the  sap  exudes  in  the  same  way, 
and  this  has  a  certain  solvent  action  upon  the  particles 
to  which  the  root-hairs  cling.  Thus  certain  salts  can  be 
absorbed,  though  they  may  be  soluble  neither  in  pure 
water  nor  in  water  containing  carbon  dioxide. 

A  similar  experiment  to  the  one  just  described  will 
demonstrate  this  property  of  the  acid  sap.  If,  instead  of 
gypsum,  a  polished  plate  of  marble  is  inserted  into  the 
flower-pot,  after  a  certain  time  of  growth  of  the  plant  con- 
tained in  it,  the  plate  will  exhibit  a  tracing  of  the  course 
of  the  roots  which  have  come  into  contact  with  it,  but, 


138  VEGETABLE  PHYSIOLOGY 

instead  of  being  in  relief  as  in  the  former  case,  it  will  be 
etched  to  a  certain  depth.  The  solvent  influence  can  thus 
be  seen  to  come  from  the  root  itself  and  not  from  the  water 
in  the  soil.  It  will,  in  fact,  be  the  acid  sap  which  makes 
its  way  out  of  the  root -hairs. 

Certain  constituents  of  the  soil  can  be  absorbed  which 
are  made  available  in  neither  of  the  ways  mentioned. 
Soils  contain  many  constituents  which  cannot  pass  through 
the  protoplasm,  but  which,  in  the  presence  of  water,  react  with 
one  another,  producing  new  compounds  which  are  capable 
of  such  osmotic  entry  and  which  are  consequently  absorbed. 

The  solutions  taken  in  are  excessively  dilute.  We  cannot 
make  a  plant  take  up  a  greater  quantity  of  any  salt  by 
bringing  its  roots  into  contact  with  a  strong  solution  of  it. 
There  is  a  certain  relation  necessary  between  the  substance 
and  the  water,  which  has  been  the  subject  of  considerable 
investigation.  For  every  salt  there  is  a  particular  concen- 
tration or  strength  of  solution,  which  if  presented  to  the 
plant  will  be  absorbed  unchanged  ;  if  the  solution  found  by 
the  roots  is  stronger  than  this,  relatively  more  water  than 
salt  will  be  taken  from  it ;  if  weaker,  relatively  more  salt 
than  water.  It  is  seldom,  therefore,  that  a  solution  is 
absorbed  without  a  certain  modification  of  its  concentration. 
Moreover,  the  optimum  concentration  of  a  solution  of  any 
salt  is  not  the  same  for  all  plants. 

In  like  manner  the  salts  which  different  plants  absorb 
vary  in  amount.  If  two  species  are  growing  in  the  same 
soil,  side  by  side,  under  exactly  the  same  conditions,  the 
amounts  of  the  several  salts  ^present  in  the  soil  which  are 
absorbed  by  the  plants  of  the  different  species  will  not 
be  the  same.  In  each  case  the  quantity  will  vary  accord- 
ing to  the  use  the  plant  can  make  of  it.  This  is  well 
illustrated  by  the  amounts  of  silica  which  can  be  taken  up 
by  grasses  and  by  leguminous  plants  respectively.  In  an 
ordinary  pasture  there  are  always  found  several  kinds  of 
grasses,  together  with  clover  and  other  allied  plants.  An 
analysis  of  these  will  show  that  the  ash  of  the  grasses  may 


ABSOKPTION  OF  FOOD  MATERIALS         139 

contain  many  times  the  percentage  of  silica  that  is  found 
in  that  of  the  leguminous  plants.  The  grasses  accumulate 
silica  in  their  epidermal  cells,  while  the  leguminous  plants 
do  not.  Hence  the  absorption  of  that  substance  soon 
ceases  in  the  latter  case. 

Again,  if  a  particular  soil  contains  several  different 
salts,  a  plant  growing  in  it  will  not  absorb  them  in  equal 
proportions,  nor  in  those  in  which  they  exist  in  the  soil. 
An  illustration  of  this  fact  is  afforded  also  by  marine  AlgaB, 
which  accumulate  in  their  tissues  much  greater  amounts 
of  potassic  than  of  sodic  salts,  though  sea-water  contains 
much  larger  quantities  of  the  latter  than  of  the  former. 
This  fact  admits  of  a  similar  explanation  to  that  given  in 
the  case  already  mentioned.  The  absorption  of  a  salt  will 
cease  as  soon  as  the  cell-sap  attains  exactly  the  same  degree 
of  concentration  as  the  entering  stream.  In  this  case 
there  will  be  no  further  osmotic  action  as  far  as  the  salt 
is  concerned,  though  there  may  be  a  continuous  entry  of 
water  into  the  absorbing  cells. 

We  have  seen  that  the  continuous  absorption  of  water 
by  the  root-hair  will  depend  upon  certain  external  condi- 
tions, such  as  the  temperature  of  the  soil,  the  activity  of 
transpiration  at  the  time,  the  degree  of  illumination  the 
plant  receives,  &c.  These  conditions  affect  also  the  absorp- 
tion of  the  substances  in  solution. 

The  substances  which  are  absorbed  by  the  roots  in  this 
way  are  naturally  very  varied.  The  most  important  of 
them  in  the  metabolism  of  the  plant  are  the  compounds  of 
nitrogen.  In  the  soil  these  exist  in  the  form  of  nitrates  or 
nitrites  of  the  metals  mentioned,  and  as  compounds  of 
ammonia.  Green  plants  take  in  little  or  none  of  the  latter, 
which  are,  however,  made  available  for  their  use  by  the 
action  of  certain  bacteria  which  the  soil  contains.  These 
humble  organisms  have  the  power  of  converting  the 
ammonia  compounds  into  nitrites,  and  the  latter  into 
nitrates,  in  which  form  they  are  taken  up.  This  process 
of  nitrification  is  the  special  property  of  two  different 


140 


VEGETABLE  PHYSIOLOGY 


bacteria,  one  of  which  forms  nitrites  from  the  ammonia 
compound,  and  the  other  transforms  nitrites  into  nitrates. 
Certain  fungi  differ  in  their  behaviour  from  green  plants, 
absorbing  ammonia  compounds  without  such  conversion. 

It  is  in  the  way  described  that  a  normal  green  plant 
absorbs  all  the  nitrogen  which  it  uses  for  the  construction 

of  food  substances.  The  nitrogen 
of  the  air  is  utilised  only  by 
its  being  made  to  enter  into 
some  form  of  combination  by 
bacteria  in  the  soil.  There  is 
much  greater  activity  in  this 
fixation  than  was  thought  till  re- 
cently, very  considerable  amounts 
of  atmospheric  nitrogen  being 
made  available  for  absorption 
by  this  instrumentality.  Certain 
lowly  Algae  are  said  to  have  the 
power  of  using  it,  but  the  process 
is  not  fully  understood.  A  few 
green  plants  can  also  use  atmo- 
spheric nitrogen,  but  their  power 
depends  upon  the  association  with 
their  roots  of  certain  fungi  or 
bacteria  which  infest  the  cortical 
tissues  and  generally  develop 
peculiar  tubercular  structures 
upon  the  roots  (fig.  82).  The 
power  was  first  observed  among 
the  members  of  the  Natural 
Order  Leguminosce,  but  it  has 
since  been  found  to  be  possessed 
by  plants  of  other  families  and  seems  to  be  more  widespread 
than  was  at  first  imagined.  The  actual  mode  of  absorption 
in  these  cases  also  is  obscure  ;  the  parts  played  by  the  root 
and  the  fungus  or  bacterium  respectively  are  not  at  all 
determined.  The  atmospheric  nitrogen  apparently  is  made 


Fia.  82.— ROOT  OF  A  LEGUMI- 
NOUS PLANT,  SHOWING  THE 
TUBERCLES  ATTACHED  TO  THE 

•  MAIN  ROOT  AND  TO  ITS 
BRANCHES. 


ABSOKPTION  OF  FOOD  MATEKIALS  141 

to  enter  into  some  form  of  combination,  and  is  then  absorbed 
by  the  root,  probably  through  the  tissue  of  the  fungus.  It 
is  not  absorbed  by  the  leaves  of  the  plant. 

Organic  compounds  of  nitrogen  are  seldom  presented  to 
the  roots  of  plants,  so  that  the  amount  of  the  element  which 
is  absorbed  in  such  a  way  is  very  small.  Indeed,  it  may 
be  said  that  such  an  absorption  is  almost  entirely  excep- 
tional. It  has  been  found  that  plants  are  able  to  utilise 
urea  and  other  amides  when  these  are  present  in  the  soil. 
In  very  rich  soils,  or  those  containing  a  large  quantity  of 
humus,  such  compounds  are  to  be  met  with,  and  there  is  a 
probability  that  they  are  more  easily  worked  up  into  actual 
nutritive  substance  than  the  inorganic  compounds  which 
have  been  spoken  of. 

Besides  compounds  of  nitrogen,  the  materials  absorbed 
by  the  roots  of  normal  green  plants  include  the  constituents 
of  the  ash.  Of  these  the  more  prominent  are  the  com- 
pounds of  potassium,  sodium,  magnesium,  calcium,  and 
iron.  The  sulphur  and  phosphorus  which  enter  into  the 
composition  of  the  protoplasm  are  also  taken  in  by  the 
roots,  in  combination  with  the  metals  mentioned,  and 
with  others  whose  occurrence  is  not  so  general.  The 
sulphur  is  absorbed  in  the  form  of  sulphates,  and  the 
phosphorus  in  that  of  phosphates,  of  these  metals. 

Potassium  is  present  in  the  soil  in  various  combinations, 
principally  as  the  sulphate,  phosphate,  chloride,  and  pro- 
bably the  silicate.  After  the  nitrate  the  chloride  appears 
to  be  the  salt  which  is  the  most  advantageous  to  plants. 
Calcium  and  magnesium  exist  in  similar  combinations,  all 
of  which,  except  the  chloride,  appear  to  be  suitable  for 
absorption.  The  chloride  is,  on  the  whole,  deleterious. 
Iron  can  be  absorbed  in  almost  any  inorganic  combination. 
Sodium  is  absorbed  in  similar  forms  to  those  of  potassium, 
the  nitrate  being  the  most  valuable.  Sodium  chloride  is 
frequently  present  in  considerable  quantity  in  the  plants 
which  are  found  on  the  sea-shore. 

Silicon    is    present    in    many    plants,    being    especially 


142  VEGETABLE  PHYSIOLOGY 

prominent  in  the  grasses  and  the  horsetails.  It  is  taken 
up  from  the  soil  in  the  form  of  soluble  silicates,  and  possibly 
to  some  extent  in  that  of  soluble  silicic  acid. 

The  other  occasional  constituents  of  the  ash,  which 
have  not  so  general  a  distribution  as  those  already  mentioned, 
include  a  number  of  metals  which  play  no  part  in  the 
nutritive  processes.  They  are  usually  present  in  very  small 
amount,  and  appear  to  be  of  accidental  occurrence,  being 
"absorbed  by  reason  of  the  solubility  of  their  salts  and 
their  power  of  entering  the  root-hairs  by  the  ordinary 
process  of  osmosis.  They  are  taken  up  in  very  various 
combinations.  Their  presence  is  not  generally  constant, 
and  appears  to  depend  entirely  on  the  composition  of 
the  soil. 

The  water  which  the  plants  take  up  is  the  chief  source 
of  the  hydrogen  and  oxygen  which  enter  into  the  com- 
position of  the  substance  of  the  plant.  A  little  of  both  these 
elements  is  taken  in  in  the  several  combinations  of  the 
metals  ;  phosphates  contain  both,  nitrates  and  carbonates 
contain  oxygen.  The  amount  of  them  absorbed  in  these 
forms  is,  however,  relatively  small.  As  we  shall  see  later, 
the  water  plays  a  very  prominent  part  in  the  constructive 
changes  which  take  place  in  the  plant. 

The  gases  present  in  the  water  of  the  soil  also  make 
their  way  into  the  root-hairs  with  the  stream,  but  the 
quantity  is  very  slight  compared  with  what  is  absorbed  by 
the  subaerial  parts.  The  gas  carbon  dioxide,  which  we 
have  seen  to  be  present  in  the  earth  in  considerable  quantity, 
is,  however,  not  made  use  of  in  the  constructive  processes. 
All  of  this  particular  food  material  is  taken  in  from  the  air. 
A  little  carbon  is  absorbed  in  the  form  of  carbonates.  Many 
complex  organic  compounds  of  carbon  are  taken  in  by  those 
roots  with  which  fungi,  such  as  the  mycorhiza  of  certain 
trees,  are  living  symbiotically,  but  this  is  exceptional.  The 
root-hairs  are  capable  of  absorbing  such  organic  compounds 
as  sugar,  but  these  materials  are  rarely  presented  to  them. 

The  absorption  of  gases  from  the  air  takes  place  in  the 


ABSOEPTION  OF  FOOD  MATEEIALS  143 

leaves  and  other  green  parts.  They  enter  freely  through 
the  stomata  so  long  as  these  are  open,  and  find  their  way 
into  the  intercellular  space  system,  the  importance  of 
which  we  have  already  examined.  These  intercellular 
spaces  contain,  as  we  have  seen,  a  mixture  of  gases  which, 
though  approximating  to  the  composition  of  the  atmo- 
sphere, yet  differs  from  it  in  the  relative  quantities  of  the 
constituents.  We  have  seen  that  the  composition  of  this 
mixture  of  gases  tends  to  become  uniform  by  the  currents 
which  circulate  in  the  intercellular  cavities,  and  by  the 
slower  processes  of  diffusion,  which  are  set  up  in  conse- 
quence of  local  production  or  abstraction  of  particular 
constituents.  So  long  as  the  stomata  and  the  lenticels  are 
open,  the  composition  of  the  atmosphere  within  the  plant 
tends  to  become  identical  with  that  of  the  external  air. 
The  actual  absorption  of  the  gases  takes  place  almost 
entirely  from  this  internal  reservoir,  very  little  finding 
entrance  into  the  cells  of  the  epidermis.  A  certain  amount 
is,  however,  taken  in  by  the  very  young  parts  which  have 
not  become  modified  by  the  development  of  a  cuticle. 

The  cells  which  abut  upon  the  spaces  in  the  leaves  and 
other  green  parts  are  those  which  are  principally  concerned 
in  the  absorption  of  gases.  Their  walls  are  very  thin  and 
delicate,  and  are  saturated  with  water.  The  different  gases 
present  dissolve  in  the  outermost  film  of  this  water,  according 
to  their  degree  of  solubility,  and  thence  diffuse  slowly  through 
the  membrane  into  the  cell-sap,  which  saturates  the  proto- 
plasm and  fills  the  vacuoles.  The  quantity  of  each  taken 
up  depends,  as  in  the  case  of  the  metallic  salts  already 
discussed,  upon  the  ability  of  the  protoplasts  to  make  use 
of  the  gas,  and  so  to  withdraw  it  from  the  sap.  If  it  can  be 
combined  in  any  way  with  other  bodies  in  the  cell,  or  with 
the  living  substance  itself,  it  is  thus  withdrawn  from  the 
water,  and  room  is  made  for  more  to  enter.  If  not,  the 
limit  of  saturation  of  the  sap  is  soon  reached. 

The  only  gas  which  is  absorbed  from  the  air  for  the 
purposes  of  food  construction  is  carbon  dioxide.  This 


144 


VEGETABLE  PHYSIOLOGY 


exists  in  the  atmosphere  in  very  small  amount,  not  quite 
four  parts  in  ten  thousand  being  normally  present.  The 
very  large  green  surface  which  an  ordinary  terrestrial  plant 
possesses  renders,  however,  a  considerable  amount  of 
absorption  possible.  Brown  and  Escombe  showed  in  1900 
that  it  is  possible  for  carbon  dioxide,  though  present  in  the 
air  in  such  minute  quantities,  to  enter  a  leaf  through  such 
small  apertures  as  stomata  in  quantities  so  great  that  a 
sunflower  is  able  to  manufacture  1*8  grain  of  carbohydrate 
per  square  metre  per  hour.  In  a  series  of  experiments  on 
the  passage  of  carbon  dioxide  through  diaphragms  pierced 


FIG.  83. — TRANSVERSE  SECTION  OF  THE  BLADE  OP  A  LEAP,  SHOWING  THE 
DIFFERENT  ARRANGEMENT  OF  THE  MESOPHYLL  ON  THE  Two  SIDES,     x  100. 

by  minute  apertures,  they  found  that,  if  the  latter  be  suffi- 
ciently small,  diffusion  takes  place  through  them  as  rapidly 
as  if  there  were  no  separating  partition  at  all.  If  the  general 
conditions  are  favourable,  the  absorption  is  continuous, 
for  carbon  dioxide  is  at  once  decomposed  or  made  to  enter 
into  some  form  of  combination  in  the  cells  of  the  green 
tissues,  and  so  a  stream  is  always  entering. 

Both  nitrogen  and  oxygen  are  soluble  in  water,  though 
to  a  different  extent.  It  has  already  been  stated  that 
the  nitrogen  so  taken  in  is  not  used  in  the  constructive 
processes,  and  accordingly  a  mere  trace  is  absorbed  in  this 
way.  A  larger  amount  of  oxygen  enters,  but  experiments 
have  proved  that  it  is  not  used  for  the  manufacture  of 
nutritive  substances,  being  applied  to  other  purposes. 


ABSORPTION  OF  FOOD  MATERIALS         145 

The  absorption  of  carbon  dioxide  takes  place  usually 
at  the  ordinary  atmospheric  pressure.  In  some  parts  of 
the  internal  reservoirs  it  exists  at  a  slightly  higher  pressure, 
in  consequence  of  a  local  production  in  the  tissues.  Plants 
can,  however,  absorb  this  gas  when  it  is  present  in  much 
larger  quantities  than  it  is  in  air.  Too  much,  however,  is 
possible,  and  then  the  cells  are  unable  to  take  it  in  at  all. 

The  continuous  absorption  of  carbon  dioxide  is  possible 
only  under  certain  conditions  ;  the  cells  which  contain 
chloroplasts  are  the  only  ones  which  can  take  it  in  any 
quantity,  and  they  can  only  do  so  when  they  are  exposed  to 
light,  preferably  that  of  bright  sunshine,  and  when  the  plant 
is  maintained  at  an  appropriate  temperature.  Its  absorption 
is  accompanied  by  the  exhalation  of  a  volume  of  oxygen 
which  is  equal  to  the  volume  of  the  carbon  dioxide  absorbed, 
and  it  is  attended  by  a  continuous  increase  in  the  weight  of 
the  plant. 

We  have  seen  that  most  of  the  water  absorbed  by  the 
roots  is  conveyed  regularly  through  the  axis  of  the  plant 
until  it  reaches  the  leaves,  in  which,  after  traversing  the 
cells  of  the  mesophyll,  it  is  evaporated  into  the  intercellular 
spaces.  Into  these  cells  of  the  interior  of  the  leaf,  all  the 
materials  used  in  the  construction  of  food  are  thus  at  once 
transported,  both  those  entering  the  tissues  from  the  soil, 
and  those  absorbed  from  the  air.  These  mesophyll  cells 
have  generally  a  different  arrangement  on  the  two  sides  of 
the  leaf  (fig.  83),  but  they  all  agree  in  containing  chloro- 
plasts. In  them  takes  place  the  work  of  construction 
of  organic  nutritive  substance,  such  as  the  plant  can  live 
upon — work  which  is  carried  out  mainly  through  the 
instrumentality  of  the  chloroplasts. 


10 


146  VEGETABLE  PHYSIOLOGY 


CHAPTEE  X 

THE  CHLOROPHYLL  APPARATUS 

The  food  materials  whose  absorption  we  have  now  discussed 
are  built  up  in  the  body  of  the  plant  into  such  substances 
as  are  capable  of  being  assimilated  by  the  protoplasm,  and 
consequently  of  ministering  to  its  nutrition.  They  undergo 
a  striking  series  of  changes  before  they  are  capable  of  sub- 
serving this  purpose,  and  of  becoming  incorporated  into 
the  plant-body.  The  great  object  to  be  attained  is  the  con- 
struction and  growth  of  the  living  substance,  which  itself 
subsequently  produces  the  more  permanent  material  that 
we  find  stored  in  the  shape  of  the  masses  of  wood  and  bark 
and  the  other  substances  which  an  adult  plant  contains. 
The  green  plant  contains  a  mechanism  for  the  formation  of 
organic  substance  from  these  simple  organic  materials, 
and  it  is  to  the  activity  of  this  mechanism  that  we  owe 
almost  the  whole  of  the  organic  matter  which  is  found  in 
nature,  whether  exhibited  by  animal  or  by  vegetable  struc- 
tures. This  mechanism  is  known  by  the  name  of  the 
chlorophyll  apparatus,  and  our  attention  must  now  be  turned 
to  its  nature  and  its  mode  of  working. 

Chlorophyll  is  a  green  colouring  matter  which  is  gene- 
rally found  associated  with  definite  protoplasmic  bodies 
known  as  plastids.  These  are  usually  considered  to  possess  a 
reticulated  structure,  and  the  pigment,  in  some  form  of  solu- 
tion, occupies  probably  the  meshes  of  the  network.  From 
their  being  coloured  green  by  the  pigment  they  are  known 
as  chloroplastids  or  chloroplasts.  The  solvent  of  the 


THE  CHLOBOPHYLL  APPAKATUS  147 

pigment  which  is  in  these  bodies  is  of  a  fatty  nature,  and 
is  probably  some  kind  of  oil.  Alcohol,  chloroform,  ether, 
benzol,  and  a  few  other  liquids  can  extract  the  chlorophyll 
from  the  plastids  and  leave  them  colourless.  The  pigment 
can  be  obtained  from  them  also  by  treatment  with  dilute 
alkalies,  such  as  potash  and  soda.  By  whatever  solvent  it 
is  extracted,  however,  it  appears  to  undergo  decomposition, 
so  that  the  solution  does  not  yield  it  up  in  the  form  in 
which  it  exists  in  the  vegetable  cell. 

A  solution  of  chlorophyll  in  alcohol  or  chloroform  shows 
the  curious  property  of  fluorescence ;  if  regarded  by  trans- 
mitted light  it  appears  green,  whatever  may  be  the  degree 
of  concentration  of  the  solution  ;  if  a  strong  solution  is 
looked  at  by  reflected  light,  it  has  a  blood-red  coloration. 

When  a  beam  of  white  light  is  allowed  to  pass  through 

a  prism,  and  is  then  made  to  impinge  upon  a  screen  of  white 

paper,  it  gives  the  appearance  of  a  band  in  which  all  the 

colours    are    represented    in    the    following    order : — red, 

orange,  yellow,  green,  blue,  indigo,  and  violet.     This  is  due 

to  the  different  degrees  in  which  the  rays  which  produce 

the  sensations  of  those  colours  are  bent  or  deflected  by 

the  prism.     This  coloured  band  is  called  the  spectrum  of 

white  light.     In  order  to  get  it  exhibited  to  the  greatest 

advantage,  it  is  best  to  admit  the  beam  of  light  to  the 

prism  through  a  narrow  slit.     The  spectrum  may  then  be 

regarded  as  a  succession  of  images  of  the  slit,  each  ray 

giving  its  own  image  of  the  aperture  and  producing  that 

image  in  its  appropriate  colour.     If  a  solution  of  chlorophyll 

is  placed  in  the  path  of  the  beam  before  it  reaches  the  slit, 

the  resulting  spectrum  is  found  to  be  considerably  modified. 

Instead  of  showing  a  continuous  band  in  which  all  the 

colours  are  represented,  it  is  interrupted  by  seven  vertical 

dark  spaces.     The  rays  which  would  have  occupied  these 

spaces  in  the  absence  of  the  solution  of  chlorophyll  have  no 

power  to  pass  through  the  latter,  and  consequently  their 

images  of  the  slit  are  represented  by  dark  lines,  which 

together    constitute    the    black    bands.     In    other    words, 

10* 


148 


VEGETABLE  PHYSIOLOGY 


chlorophyll  absorbs  these  particular  rays  of  light  which  are 
missing. 

In  fig.  84  is  a  representation  of  the  spectrum  which 
such  treatment  produces  and  which  is  called,  from  the  facts 
just  narrated,  the  absorption  spectrum  of  chlorophyll.  The 
uppermost  figure  is  that  which  is  exhibited  by  an  alcoholic 
solution  or  extract  of  leaves  ;  the  middle  one  is  given  by 
chlorophyll  dissolved  in  benzol.  The  first  band  on  the  left 
is  the  darkest,  and  is  found  to  be  in  the  red  part  of  the  spec- 
trum. The  three  bands  on  the  right  are  broader,  but  are 


W  V  M 

FIG.  84. — ABSORPTION  SPECTRA  OF  CHLOROPHYLL  AND 
XAKTHOPHYLL.    (After  Kraus.) 


T2f 


not  so  well  defined.  They  cover  nearly  all  the  blue  end. 
The  three  thinner  and  lighter  bands  are  in  the  yellow  and 
green  parts  of  the  spectrum.  Chlorophyll  therefore  has 
the  power  of  absorbing  a  large  number  of  red  rays,  a  good 
many  blue  and  violet  ones,  and  a  few  of  the  green  and  yellow. 
The  distinctness  with  which  these  absorption  bands  are 
seen  depends  upon  the  strength  of  the  solution,  those  in  the 
red  and  blue  being,  however,  always  prominent.  Careful 
experiments  have  proved  that  chlorophyll  is  a  single  pigment 
and  not  a  mixture  of  two,  as  has  often  been  stated.  It  is, 
however,  easily  decomposed,  and  the  products  of  its  decom- 


THE  CHLOROPHYLL  APPAEATUS  149 

position  are  generally  found  with  it  in  the  chloroplast.  One 
of  these,  Xanfhopliyll,  which  is  of  a  bright  yellow  colour,  is 
always  extracted  with  the  chlorophyll  by  alcohol.  It  can 
be  separated  from  the  extract  by  appropriate  means,  and 
its  solution  yields  the  absorption  spectrum  represented 
below  those  of  chlorophyll  in  fig.  84.  Another  pigment, 
Eryihrophytt,  is  present  in  those  leaves  which  are  found  upon 
the  trees  in  autumn.  Like  xanthophyll,  it  appears  to  be 
a  product  of  the  decomposition  of  chlorophyll,  and  it  has  a 
spectrum  which  differs  from  both  the  others. 

It  is  extremely  difficult  to  say  what  is  the  chemical 
composition  of  chlorophyll,  on  account  of  the  readiness  with 
which  it  is  decomposed.  In  all  the  processes  which  have 
been  adopted  for  its  extraction  it  undergoes  decomposition, 
and  consequently  no  definite  conclusions  as  to  its  chemical 
nature  can  at  present  be  arrived  at.  It  can  be  made  to 
yield  definite  crystals  by  appropriate  methods  of  treatment 
after  extraction,  but  it  is  probable  that  these  crystals  are 
a  derivative  of  chlorophyll  and  not  the  pure  pigment. 
Analyses  of  the  crystals  have  been  made  by  Gautier  and  by 
Hoppe-Seyler,  who  give  them  the  following  percentage 
compositions  : — 

Gautier.  Hoppe-Seyler. 

C      73-97  73-34 

H      9-8  9-72 

N      4-15  5-68 

0    10-33  9-54 

Ash  1-75  1-72 

According  to  Hoppe-Seyler  the  ash  contains  phosphorus 
and  magnesium. 

From  his  analysis  Gautier  came  to  the  conclusion  that 
chlorophyll  is  related  to  the  colouring  matter  of  the  bile  ; 
Hoppe-Seyler  considered,  on  the  other  hand,  that  it  is  a 
fatty  body  allied  to  lecithin. 

Wilstatter  found  that  the  phosphorus  of  the  ash  is  an  acci- 
dental impurity,  and  hence  that  chlorophyll  is  not  related 


150  VEGETABLE  PHYSIOLOGY 

to  lecithin.     He  held  it  to  be  an  ester  of  an  unsaturated 
alcohol  to  which  he  gave  the  name  phytol  (C20  H400). 

Except  in  the  lowest  unicellular  plants,  the  chlorophyll  is 
always  attached  to  some  form  of  protoplasmic  body  or 
plastid.  These  are  small  masses,  of  varying  size  and  shape, 
which  are  embedded  in  the  general  cytoplasm  of  the 
cell  (fig.  85).  Even  in  the  lowliest  plants  it  is  apparently 
never  uniformly  distributed  through  the  body  of  the  proto- 
plast. The  form,  dimensions,  and  structure  of  the  chloro- 
plast  differ  considerably  in  the  different  groups  of  plants. 
In  some  of  the  filamentous  green  seaweeds  it  may  appear 
as  variously  shaped  bands  or  plates.  Spirogyra  shows  it 
as  a  spiral  band  passing  round  the  cell ;  in  Zygnema  it  has 
the  form  of  two  star-shaped  masses  which  are  attached 
to  the  cytoplasm  by  bridles  extending 
to  the  cell- wall.  In  the  brown  and 
red  seaweeds  the  plastids  are  not  green, 
but  have  the  appropriate  colours  of  the 
plants.  These  plastids  contain  other 
pigments  in  addition  to  the  chlorophyll, 
but  the  latter  can  be  made  apparent 
by  extracting  the  cells  with  cold  dis- 
tilled water,  in  which  the  other  pigments 
FIG.  85.-CHLOROPLASTS  are  soluble.  In  all  plants  higher  in  the 

EMBEDDED  IN  THE  PfiO-  -  *,  . 

TOPLASM  OF  A  CELL  OP    scale  than  the  Algae  the  chloroplasts  are 

THE  PALISADE  TISSUE    found  as  round  or  oval  bodies  embedded 

in  the  cytoplasm.     They  never  occur 

in  the  vacuoles  of  the  cells.      Though  normally  green,  they 

can  assume  other  colours,  such  as  yellow,  brown,  or  red,  but 

this  is  due  to  an  alteration  of  the  pigment  they  contain. 

Examples  of  this  change  are  afforded  by  the  assumption 

of  the  autumnal  tints  by  foliage  leaves,  and  by  the  changes 

in  colour  which  are  characteristic  of  ripening  fruits. 

In  the  Mosses  the  chloroplasts  are  found  throughout 
the  cells  of  the  leaves,  in  the  outer  parts  of  the  sporogonium, 
and  in  certain  cells  of  the  axis.  In  the  Ferns  they  occur 
chiefly  in  the  leaves,  occupying  the  cells  of  the  epidermis  as 


THE  CHLOEOPHYLL  APPAEATUS          151 

well  as  those  of  the  mesophyll.  In  the  higher  plants  they 
are  found  most  prominently  in  the  mesophyll  of  the 
leaves,  the  epidermis  as  a  rule  being  free  from  them. 
When  the  leaves  are  dorsiventral  in  structure,  the 


s.pa 


Fio.  86.— TRANSVERSE  SECTION  OP  PORTION  OP  THE  BLADE  OP  THE 
LEAP  OP  Beta. 

cu,  cuticle ;   ep,  epidermis  ;  p.pa,  palisade  tissue  ;  s. pa.  spongy  tissue ; 
v.b,  vascular  bundle  ;  st,  stoma ;   i.a,  intercellular  space. 


chloroplasts  are  more  numerous  in  the  palisade  paren- 
chyma which  lies  just  below  the  upper  epidermis  than  they 
are  in  the  spongy  tissue  which  occupies  the  lower  half  of 
the  thickness  of  the  leaf  (fig.  86).  The  guard-cells  of  the 
stomata,  however,  always  contain  them.  The  green  cortex 
of  young  stems  and  twigs  also  exhibits  them.  In  such 


152 


VEGETABLE  PHYSIOLOGY 


plants  as  the  Casuarinas  and  the  Equisetums  (fig.  87),  in 
which  the  leaves  are  rudimentary,  definite  longitudinal 
bands  of  cells  in  the  young  stems  contain  them. 

The  structure  of  such  a  chloroplast  as  is  characteristic 
of  one  of  the  higher  plants  has  not  been  very  completely 
investigated.  ^There  is  undoubtedly  a  protoplasmic  basis 


FIG.  87. — TRANSVERSE  SECTION  OF  PORTION  OF  AERIAL  STEM  OF  AN  Equisetum. 
a,  cortical  lacuna ;    6,  lacuna  in  vascular  bundle ;    c,  chlorophyll-containing  cells. 


with  which  the  colouring  matter  is  in  some  way  associated. 
As  already  stated,  many  botanists  consider  the  protoplasm 
to  be  arranged  in  a  network,  whose  meshes  are  filled  with 
a  solution  of  the  pigment.  Others  consider  the  protoplasm 
to  be  homogeneous,  but  honeycombed  with  vacuoles  which 
are  filled  with  the  solution  of  the  chlorophyll.  Others  again 
think  that  the  pigment  forms  a  layer  round  the  plastid.  By 
the  action  of  dilute  acids,  or  by  treating  the  chloroplasts 


THE  CHLOEOPHYLL  APPAEATUS  153 

with  steam,  the  colouring  matter  may  be  made  to  exude 
from  the  framework  in  viscid  drops,  leaving  the  latter  colour- 
less. It  then  appears  to  have  a  reticular  structure,  but 
how  far  this  condition  is  brought  about  by  the  action  of 
the  reagents  is  uncertain.  The  chlorophyll,  however,  is  cer- 
tainly not  uniformly  diffused  through  the  body  of  the  plastid. 

In  the  process  of  the  formation  of  the  chloroplast  it  is 
not  difficult  to  see  that  its  two  constituents  are  not  in- 
extricably connected.  The  plastids  are  not,  as  already 
mentioned,  differentiated  out  of  the  ordinary  protoplasm  of 
the  cell,  but  are  formed  by  the  division  of  other  plastids. 
In  many  cases  they  are  found  without  the  colouring  matter, 
as  in  underground  organs  such  as  the  tubers  of  the  potato. 
They  are  then  known  as  leucoplasts.  Plants  which  are 
grown  in  darkness  have  no  green  colouring  matter  in 
their  leaves,  but  the  cells  of  their  mesophyll  contain  the 
plastids  much  as  normal  ones  do.  They  are  pale  yellow 
in  colour,  containing  another  pigment  known  as  etiolin, 
which  is  replaced  by  chlorophyll  when  the  leaf  is  brought 
into  the  presence  of  sunlight.  Exposure  to  light  is  almost 
universally  a  necessary  condition  for  the  formation  of  the 
green  pigment.  Exceptions  are  known  among  the  Ferns 
and  the  Conifers,  particularly  the  seedlings  of  Pinus  ;  also 
in  the  seed  of  Euonymus  europceus,  the  embryo  of  which  is 
green,  though  it  is  buried  in  the  interior  of  the  endosperm 
and  surrounded  by  a  thick  testa  covered  by  an  arillus. 
If  a  green  stem  is  withdrawn  from  the  light,  the  chloro- 
phyll slowly  disappears,  as  is  shown  in  the  process  of  the 
bleaching  of  celery.  The  disappearance  is,  however,  very 
gradual.  It  is  probable  that  in  the  living  chloroplast  the 
colouring  matter  is  continually  being  decomposed  and 
reconstructed,  and  that  the  reason  of  the  bleaching  is  that 
the  reconstruction  cannot  take  place  in  darkness.  Light  of 
too  great  intensity  causes  the  destruction  of  the  green  colour. 

Chlorophyll  can  be  developed  only  when  the  temperature 
rises  above  a  certain  point,  which  varies  with  different  plants. 
It  is  a  matter  of  common  observation  that  the  leaves  of  young 


154  VEGETABLE  PHYSIOLOGY 

Hyacinths  which  emerge  from  the  soil  in  the  early  spring  are 
often  colourless  or  pale  yellow.  The  chloroplasts  are  found 
to  he  present  in  such  leaves,  but  they  are  yellow,  owing  to 
the  presence  of  etiolin  instead  of  chlorophyll.  The  leaves 
which  are  produced  later,  when  the  temperature  of  the  air 
is  higher,  have  the  normal  green  appearance. 

Chlorophyll  is  not  developed  in  a  plant  unless  the  latter 
is  supplied  with  a  certain  quantity  of  iron,  but  the  relation 
of  the  latter  to  the  pigment  is  not  known.  It  apparently 
does  not  enter  into  its  composition.  The  influence  of  the 
metal  can  be  ascertained  by  cultivating  a  seedling,  by  the 
method  of  water-culture,  in  a  solution  which  is  free  from 
iron.  The  seedling  assumes  a  sickly  yellow  appearance, 
not  unlike  that  presented  by  a  plant  grown  in  darkness. 
It  is  said  to  be  chlorotic.  The  addition  of  a  very  small 
quantity  of  an  iron  salt  to  the  culture-medium  causes  the 
appearance  of  chlorophyll  in  the  plastids.  The  presence  of 
oxygen  is  also  necessary  for  the  formation  of  the  pigment. 

The  chlorophyll  apparatus  of  a  plant  is  primarily  con- 
cerned with  the  production  of  carbohydrate  substances,  such 
as  the  various  sugars  which  the  plant  contains,  and  it  is  to 
the  formation  of  these  that  attention  must  first  be  given. 
It  carries  out  this  constructive  process  only  under  particular 
conditions,  the  most  important  of  which  is  light.  We  have 
seen  that  a  certain  degree  of  illumination  is  necessary  for 
the  lormation  of  the  chlorophyll.  The  pigment  once 
formed  may  continue  to  exist  for  a  time  in  darkness,  but  it 
is  quite  incapable  of  exercising  any  constructive  power 
unless  light  be  admitted  to  it.  Consequently  the  formation 
of  carbohydrates  is  an  intermittent  process,  being  quite  in 
abeyance  during  the  night.  The  effect  of  light  is  thus 
twofold,  its  access  causing  the  original  formation  and  sub- 
sequently the  activity  of  the  chlorophyll  apparatus.  The 
illumination  need  not  be  very  intense,  though  it  is  probable 
that  the  greatest  activity  is  manifested  in  a  bright  diffused 
light.  Plants  which  grow  even  in  deep  shade  are,  however, 
capable  of  forming  carbohydrates.  It  must  be  remembered, 


THE  CHLOKOPHYLL  APPAKATUS     155 

too,  that  the  chloroplasts  are  situated  some  little  distance 
within  the  leaf  or  stem,  at  any  rate  in  phanerogamic  plants, 
and  there  must  be  a  certain  loss  of  light  as  it  penetrates 
through  the  epidermis. 

The  activity  of  the  chlorophyll  apparatus  is  also  con- 
siderably influenced  by  variations  of  temperature.  The 
lower  limit  beyond  which  no  carbohydrates  are  constructed 
lies  probably  a  little  below  the  freezing-point  of  water,  at 
which  point,  however,  activity  is  not  long  maintained,  and 
then  only  by  alpine  forms.  Jumelle  has  stated  that  in  cer- 
tain plants  of  hardy  type  it  can  proceed  at  as  low  a  tempera- 
ture as  —  40°  C.  Plants  which  normally  live  in  hot  climates 
cannot  manifest  any  power  of  action  below  about  4°  C. 
The  optimum  temperature  for  the  plants  of  temperate 
climates  is  from  15°  C.  to  25°  C.,  above  which  activity 
diminishes,  though  not  very  rapidly,  ceasing  when  about 
45°  C.  is  reached.  These  high  temperatures  affect  the 
living  substance  of  the  chloroplasts  very  injuriously. 

The  activity  of  the  chlorophyll  apparatus  is  dependent 
also  to  some  extent  upon  certain  of  the  mineral  salts  present 
in  the  cells.  According  to  Bokorny,  it  cannot  be  called 
into  play  in  the  absence  of  compounds  of  potassium. 

As  the  activity  of  the  chlorophyll  apparatus  is  so  essentially 
dependent  upon  light,  the  process  of  construction  of  carbo- 
hydrate substances  from  carbon  dioxide  and  water,  which 
is  its  primary  object,  may  appropriately  be  called  photo- 
synthesis. This  term  has  certain  advantages  over  the  older 
expression,  the  assimilation  of  carbon  dioxide,  as  the  term  '  assi- 
milation' may  preferably  be  reserved  for  the  process  of  the 
incorporation  of  food  into  the  substance  of  the  protoplasm. 

Photosynthesis  consists,  then,  in  the  formation  of  some 
form  of  carbohydrate  from  the  carbon  dioxide  which  is 
absorbed  from  the  air,  and  the  water  which  is  present  in 
the  cells.  When  these  simple  bodies  are  exposed  to  the 
action  of  the  chloroplast  in  presence  of  light  and  mode- 
rate warmth,  the  carbon  dioxide  disappears,  and  a  volume 
of  oxygen  equal  to  that  of  the  carbon  dioxide  is  exhaled. 


156 


VEGETABLE  PHYSIOLOGY 


The  apparatus  shown  in  fig.  88  will  enable  this  inter- 
change of  gases  to  be  seen.  Into  a  glass  jar  is  poured  some 
water  containing  carbon  dioxide  in  solution.  Some  aquatic 
plant  is  put  into  the  water  and  a  funnel  inserted  above  it, 
the  end  of  which  rises  into  a  burette  filled  with  water  and 
closed  by  a  stopcock.  The  whole  apparatus  being  placed 
in  sunlight,  bubbles  of  oxygen  will  be  given  off  by  the 
leaves  and  will  rise  into  the  burette.  If  no  carbon  dioxide 
is  in  the  water,  no  oxygen  will  be  given  off. 

There  is  little  certainly  known  at  present  as  to  the  details 
of  the  changes  which  connect  these  two  phenomena.  It 
has  been  suggested  by  Baeyer  that  the  carbon  dioxide  is 
decomposed  with  the  formation  of  carbon  monoxide  and 
oxygen,  according  to  the  equation  2COo  =  2CO  +  Og.  At 
the  same  time  there  is  a  decomposition  of  water,  possibly 
in  the  way  denoted  by  the  equation  2H20  =  2H2  +  Og- 
The  oxygen  is  given  off,  the  volume  being  found,  when  care- 
fully measured,  to  be  equal 
to  the  volume  of  carbon 
dioxide  undergoing  de- 
composition. The  carbon 
monoxide  and  the  hydro- 
gen are  then  thought  to 
unite,  producing  form- 
aldehyde, a  body  repre- 
sented by  the  formula 
CHoO,  or  preferably 
1  HCOH.  This  suggested 
series  of  reactions  agrees 
fairly  closely  with  the  ob- 
served facts,  but  it  must 

not  be  regarded  as  anything  more  than  an  hypothesis.  Indeed 
there  are  considerable  difficulties  in  accepting  it  as  it  stands. 
There  is  no  evidence  that  carbon  monoxide  is  formed. 
Experiments  have  shown  that  this  gas  is  quite  useless  to 
most  plants  ;  if  it  is  supplied  in  the  place  of  the  dioxide, 
the  formation  of  carbohydrates  does  not  take  place.  Nor  has 


FIG.  88. — APPARATUS  TO  SHOW  THE  EVOLU- 
TION OF  OXYGEN  BY  A  GREEN  PLANT 
IN  SUNLIGHT. 


THE  CHLOKOPHYLL  APPAKATUS  157 

any  formation  or  liberation  of  hydrogen  ever  been  detected  so 
long  as  the  plant  is  maintained  in  normal  conditions. 

The  formation  of  formaldehyde,  again,  is  very  difficult 
of  proof.  It  very  readily  undergoes  change,  and  therefore 
is  difficult  to  detect  in  a  plant.  It  has  been  found,  how- 
ever, that  if  Spirogyra  is  fed  with  a  compound  of  form- 
aldehyde and  sodium-hydrogen-sulphite,  which  slowly 
evolves  the  former  in  the  presence  ot  water,  a  formation 
of  carbohydrates  occurs.  This  cannot,  however,  be  accepted 
as  proof  that  formaldehyde  normally  subserves  this  purpose. 

There  is,  however,  a  certain  amount  of  evidence  that 
formaldehyde  plays  some  part  in  photosynthesis.  Bouilhac 
and  Treboux  have  succeeded  in  getting  plants  to  grow  in 
a  very  dilute  solution  of  it.  Moreover,  formaldehyde  has 
been  obtained  from  plants  by  distilling  leaves  which  have 
been  exposed  for  a  long  time  to  light  and  subsequently 
soaked  in  water.  Even  in  these  experiments,  however,  it 
is  not  certain  how  it  was  produced.  In  any  except  very 
dilute  solutions  it  is  intensely  poisonous  to  plants. 

Within  the  last  few  years  formaldehyde  has  been  detected 
in  leaves  which  have  been  plucked  after  exposure  for  some 
hours  to  a  bright  sun.  The  test  was  devised  by  Mulliken, 
Brown,  and  French,  and  is  extremely  delicate.  To  apply  it 
take  1  c.c.  of  a  5  per  cent,  solution  of  gallic  acid  in  absolute 
alcohol,  add  about  3  c.c.  of  pure  concentrated  sulphuric 
acid  so  that  the  two  fluids  do  not  mix,  and  allow  a  small 
quantity  of  the  suspected  extract  of  the  leaves  to  stream 
down  the  side  of  the  test  tube  in  which  the  experiment  is 
being  conducted.  The  presence  of  formaldehyde  will  be 
indicated  by  the  formation  of  a  blue-green  ring  at  the  zone 
of  contact  of  the  upper  and  lower  liquids. 

If  we  concede  that  formaldehyde  is  very  probably  the 
first  stage  in  the  photosynthetic  process,  a  consideration  of 
the  probable  decomposition  seems  to  lead  us  to  the  view 
that  the  carbon  dioxide  and  the  water  are  made  to  interact 
without  the  liberation  of  carbon  monoxide,  and  that  the 
reaction  may  be  represented  by  the  equation  C03  +  H30  = 


158  VEGETABLE  PHYSIOLOGY 

HCOH  +  02,  which  agrees  equally  well  with  the  observed 
facts. 

The  formaldehyde  may  give  rise  without  much  difficulty 
to  a  form  of  sugar.  It  is  a  property  of  the  aldehydes  to 
undergo  readily  what  is  known  as  polymerisation,  or  con- 
densation of  several  molecules.  Such  a  condensation 
of  formaldehyde  would  lead  to  the  formation  of  sugar 
thus  : — 6HCOH  =  C6Hi206.  There  are  many  sugars  of 
"this  composition  in  the  plant,  especially  glucose  or  grape 
sugar,  and  fructose  or  fruit  sugar. 

That  some  such  process  takes  place  is  extremely  probable, 
for  sugar  is  present  in  the  mesophyll  cells  very  speedily 
after  the  absorption  of  the  carbon  dioxide  and  the  begin- 
ning of  the  exhalation  of  oxygen.  Sugar  of  some  kind 
appears  to  be  the  first  carbohydrate  to  be  formed  ;  it  is  not 
very  readily  detected,  being  freely  soluble  in  the  cell-sap. 
Almost  as  quickly  as  the  formation  of  sugar  we  have  the 
appearance  of  starch  in  the  substance  of  the  chloroplasts, 
and  as  this  is  easily  visible,  it  was  long  thought  that  starch 
was  the  culminating  product  of  the  photosynthetic  process. 
We  shall  find  reasons  shortly  for  suggesting  a  wholly  different 
meaning  to  the  appearance  of  the  starch,  that  it  is  indeed 
only  a  temporary  store  of  carbohydrate  in  an  insoluble 
condition,  due  to  the  production  of  sugar  being  in  excess 
of  the  quantity  which  the  cell  can  dispose  of  by  immediate 
consumption  or  translocation. 

If  we  accept  the  view  of  the  polymerisation  of  formalde- 
hyde to  give  rise  to  the  sugar,  we  cannot  withdraw  this 
operation  also  from  the  activity  of  the  chloroplast.  Sugars 
are  what  are  called  optically  active  compounds  :  that  is,  they 
possess  the  power  of  deflecting  a  ray  of  polarised  light  to 
the  right  or  to  the  left  as  the  latter  is  made  to  pass  through 
either  crystals  or  a  solution  of  them.  Formaldehyde  has  no 
such  power.  There  is  no  process  known  by  which  an 
optically  active  compound  is  formed  from  an  optically 
inactive  one  without  the  intervention  of  living  substance. 
Consequently  we  must  suppose  that  the  polymerisation  is 


THE  CHLOBOPHYLL  APPAKATUS          159 

brought  about  by  the  chloroplast  as  certainly  as  is  the 
original  change  of  the  carbon  dioxide. 

We  have  so  far  assumed  that  a  sugar  having  the  formula 
C6H1206,  and  known  as  a  hexose,  is  the  first  carbohydrate 
formed.  This,  however,  is  not  certain.  Some  experi- 
ments carried  out  in  1892  by  Brown  and  Morris  point 
rather  to  cane-sugar  as  the  first  carbohydrate  synthesised. 
Cane-sugar  is  a  more  complex  substance,  and  has  the 
formula  C12H23On.  This  conclusion  was  based  on  repeated 
observations  that  when  leaves  of  Tropceolum  were  plucked 
and  then  exposed  to  sunlight  for  twelve  hours,  there  was  a 
great  accumulation  of  this  sugar  in  the  leaf,  while  the 
simpler  hexoses  did  not  increase  in  quantity.  The  severance 
of  the  leaves  from  their  stems  prevented  the  transport  of 
the  sugars  to  any  other  part  of  the  plant,  so  that  they  accumu- 
lated at  the  seat  of  their  formation. 

Further  investigations  on  this  point  are,  however,  necessary 
before  a  definite  conclusion  can  be  arrived  at.  It  is  not 
impossible  that  the  cane-sugar  of  Tropaeolum  may  be  a 
form  of  stored  material  temporarily  deposited  as  starch  is. 
This  theory  of  the  processes  of  photosynthesis  is  by  no 
means  the  only  one  which  has  been  advanced,  though  on 
the  whole  it  is  that  which  has  been  received  with  most 
favour.  A  modification  of  Baeyer's  view  was  advanced  by 
Erlenmeyer,  who  suggested  that  the  first  interaction  of 
carbon  dioxide  and  water  leads  to  the  formation  of  formic 
acid  and  hydrogen  peroxide,  according  to  the  equation 
C02  +  2H  0  =  HCOOH  +  H302,  and  that  then  they  are 
decomposed,  yielding  formaldehyde  and  water,  and  giving 
off  oxygen,  HCOOH  +  H202  =  HCOH  +  H20  -f  02. 

A  theory  of  a  totally  different  nature  was  advanced  some 
years  ago  by  Vines.  Starting  with  the  observation  that 
a  carbohydrate  substance  (cellulose)  is  produced  or  secreted 
by  protoplasm  in  the  process  of  the  thickening  of  cell-walls, 
and  noticing  the  formation  of  starch  grains  in  the  chloroplast 
almost  as  soon  as  the  photosynthesis  has  been  established, 
he  argued  that  the  carbohydrate  is  not  directly  formed 


160  VEGETABLE  PHYSIOLOGY 

from  the  simple  materials  absorbed,  but  appears  as'a  secretion 
product  of  the  chloroplast.  He  suggested  that  a  body  possibly 
allied  to  formaldehyde  is  first  formed  according  to  Baeyer's 
theory,  and  that  this  is  used  in  the  construction  of  protein, 
by  combining  with  the  nitrogen  and  sulphur  absorbed  in  the 
form  of  salts  from  the  soil,  or  with  nitrogenous  substances 
derived  from  previous  decompositions  of  protein.  This 
protein  then  is  assimilated  by  the  protoplasm  of  the  chloro- 
plast, and  from  the  latter  the  carbohydrate  (starch)  is 
secreted. 

This  view,  while  no  doubt,  in  the  main,  accurate  as  far 
as  the  mode  of  formation  of  starch  is  concerned,  cannot  be 
regarded  as  explaining  the  formation  of  carbohydrates  from 
the  simple  compounds  absorbed.  The  leucoplast  of  the 
tuber,  as  well  as  the  chloroplast  itself  under  certain  con- 
ditions, can  form  starch  grains  when  supplied  with  sugar 
in  the  absence  of  carbon  dioxide,  and  in  all  probability  the 
appearance  of  the  starch  is  the  result  of  the  presence  of  an 
excess  of  sugar  in  the  leaf -cells.  Kegarded  as  an  explanation 
of  the  photosynthesis  of  carbohydrates,  it,  like  the  others, 
must  remain  hypothetical.  Moreover  it  is  based  upon  the 
assumption  that  starch  is  the  highest  term  reached  in  the 
plant  in  the  series  of  carbohydrate  bodies.  This  assump- 
tion, however,  is  not  supported  by  the  evidence  at  our  com- 
mand, the  construction  of  sugar  and  not  starch  being  the 
completion  of  the  photosynthetic  process  of  the  chlorophyll 
apparatus.  Though  starch  is  a  very  general  accompaniment 
to  this  process,  it  never  appears  till  a  certain  amount  of  sugar 
has  been  formed,  and  in  many  plants,  particularly  the 
onion  and  certain  other  Monocotyledons,  it  is  not  produced 
at  all,  however  active  photosynthesis  may  be.  To  this 
point  we  shall  return  in  a  subsequent  chapter. 

Another  hypothesis  of  carbohydrate  formation  was  put 
forward  in  1906  by  Usher  and  Priestley.  They  claimed 
to  have  found  that  the  interaction  of  carbon  dioxide  and 
water  leads  to  a  coincident  formation  of  formaldehyde 
and  hydrogen  peroxide.  The  latter  was  stated  to  be  at  once 


THE  CHLOROPHYLL  APPARATUS     161 

decomposed  by  an  enzyme  into  water  and  oxygen.  The 
first  decompositions  were  held  to  be  effected  by  the  light  and 
the  colouring  matter,  the  body  of  the  plastid  taking  part 
only  in  the  subsequent  constructive  processes. 

The  most  recent  theory  of  the  process  of  photosynthesis 
was  advanced  by  Harvey-Gibson  in  1907.  He  suggested 
that  the  rays  of  solar  energy  absorbed  by  the  chlorophyll 
are  transformed  into  electric  energy,  and  that  by  means 
of  the  consequent  currents .  of  electricity,  moist  carbon 
dioxide  is  decomposed,  the  result  being  the  formation  of 
formaldehyde  and  oxygen.  The  process  takes  place  in  the 
intercellular  spaces  of  the  leaf;  the  formaldehyde  is  sub- 
sequently absorbed  into  its  cells,  where  it  undergoes  poly- 
merisation, as  Baeyer  suggested. 

Though  the  production  of  starch  is  apparently  not  the 
ultimate  aim  of  the  photosynthetic  processes,  its  ready 
occurrence  affords  us  an  easy  method  of  demonstrating  the 
activity  of  the  chlorophyll  apparatus.  If  a  leaf  is  partially 
covered  by  a  piece  of  opaque  material,  and  is  then  exposed 
to  the  light,  starch  rapidly  appears  in  the  illuminated 
portion,  as  can  be  shown  by  bleaching  the  leaf  with  boiling 
alcohol,  and  then  immersing  it  in  iodine,  which  forms  a 
blue  colour  with  starch.  The  blue  tint  only  appears  where 
the  light  has  reached  the  chlorophyll  apparatus. 

These  processes  are  carried  out  by  the  chlorophyll  apparatus 
under  the  conditions  set  forth.  It  is  evident  that  such 
changes  as  have  been  described  cannot  be  accomplished 
without  the  expenditure  of  a  considerable  amount  of  energy. 
In  this  need  we  have  the  explanation  of  the  composite 
nature  of  the  chloroplast.  The  chlorophyll  absorbs  certain 
rays  of  light  which  fall  upon  it,  and  the  energy  which  is 
liberated  by  the  extinction  of  their  vibrations  is  taken  up  by 
the  protoplasm  of  the  plastid  and  applied  by  it  to  effect  the 
decompositions  that  take  place.  A  very  ingenious  method 
of  demonstrating  that  the  energy  is  derived  from  the  rays  of 
light  absorbed  by  the  pigment  was  devised  by  Engelmann. 
He  observed  that  certain_bacteria  were  excited  to  active 

11 


162  VEGETABLE  PHYSIOLOGY 

movement  only  in  the  presence  of  free  oxygen.  He  placed 
a  filament  of  a  green  alga  upon  a  glass  slide  in  a  fluid  con- 
taining a  number  of  the  bacteria,  covered  it  with  a  glass 
coyer-slip,  and  sealed  it  with  wax.  He  kept  it  in  darkness 
till  the  microbes  had  come  to  rest,  and  then  by  the  aid  of  a 
microspectroscope  he  threw  an  image  of  the  solar  spectrum 
upon  the  filament  and  observed  in  what  parts  of  it  the 
bacteria  accumulated  as  soon  as  they  began  to  move.  These 
places  corresponded  with  the  positions  of  the  absorption 
bands  which  we  have  seen  to  be  characteristic  of  the  chloro- 
phyll spectrum,  the  maximum  effect  being  produced  by  the 
deep  band  in  the  red  region.  These  were  evidently  the 
places  at  which  the  chlorophyll  apparatus  of  the  filament 
was  at  work,  the  movements  of  the  bacteria  showing  that 
oxygen  was  liberated  there.  Timiriazeff  proved  the  same 
thing  by  throwing  the  spectrum  of  solar  light  upon  a  darkened 
leaf,  when  he  found  that  starch  was  produced  only  in  the 
positions  of  those  same  absorption  bands,  indicating  that 
those  were  the  only  places  of  photosynthetic  activity. 

The  process  of  photosynthesis  has  been  found  to  proceed 
under  certain  circumstances  in  light  which  is  too  feeble  in 
intensity  to  cause  the  development  of  chlorophyll.  It  is 
effected  in  these  cases  by  the  etiolin,  which  we  have  seen 
to  be  present  under  such  conditions.  The  photosynthetic 
power  of  etiolin  is,  however,  exceedingly  small. 

The  percentage  of  carbon  dioxide  admitted  to  the  chloro- 
plasts  has  some  influence  upon  the  activity  of  the  process. 
Normal  air  contains  a  mere  trace  of  the  gas,  less  than  3  parts 
in  10,000.  A  more  copious  supply  is,  however,  distinctly 
advantageous,  and  the  activity  increases  as  the  percentage 
rises,  though  the  plant  does  not  gain  in  weight  proportion- 
ately. The  optimum  quantity  appears  to  be  about  10  per 
cent,  with  light  of  the  ordinary  intensity.  More  than  this 
gradually  exerts  a  paralysing  influence  on  the  chloroplast, 
and  consequently  sets  up  an  inhibition  of  the  apparatus. 
The  optimum  amount  of  carbon  dioxide  varies,  however, 
considerably  with  the  intensity  of  the  illumination  and 


THE  CHLOROPHYLL  APPARATUS     163 

the  temperature.  Inhibition  can  be  caused  also  by  the 
accumulation  of  the  products  of  the  activity  of  the  plastide, 
a  concentration  of  the  sugar  amounting  to  8  per  cent,  being 
sufficient  to  bring  it  about. 

The  mechanism  is  an  exceedingly  delicate  one  and  can 
be  thrown  out  of  gear  by  various  external  agencies.  Ewart 
has  shown  that  it  can  be  inhibited  by  heat,  cold,  desiccation, 
partial  asphyxiation,  prolonged  insolation,  and  by  the  action 
of  dilute  alkalies  or  mineral  acids. 

We  mentioned  at  the  commencement  of  this  chapter 
that  the  chlorophyll  apparatus  is  concerned  in  the  manu- 
facture of  almost  the  whole  of  the  organic  material  of  the 
globe.  In  a  few  humble  organisms  the  construction  of 
such  material  can  proceed  without  its  help.  These  are 
certain  bacteria  which  can  transform  ammonia  compounds 
into  salts  of  nitrous  and  nitric  acids,  growing  and  multi- 
plying at  the  expense  of  the  products  they  thus  obtain, 
together  with  carbon  dioxide.  There  are  two  kinds  of 
these  bacteria,  one  of  which  oxidises  ammonia  to  nitrous 
acid  and  the  other  converts  this  into  nitric  acid.  They 
grow  freely  in  the  soil  and  multiply  with  considerable 
rapidity,  the  result  being  the  formation  of  certain  quanti- 
ties of  organic  substance.  They  cause  the  carbon  dioxide  to 
enter  into  combination,  this  gas  being  normally  the  only 
source  of  their  supply  of  carbon.  They  possess  no  chlorophyll 
and  consequently  cannot  utilise  directly  the  radiant  energy 
of  the  sun.  Their  energy  is  apparently  derived  from  the 
oxidation  of  the  nitrogenous  compounds  which  they  attack. 
Nothing  is  known  at  present  of  the  steps  by  which  the 
synthesis  of  the  organic  matter  takes  place. 

A  process  which  at  first  appeared  to  involve  a  mechanism 
resembling  that  of  the  chlorophyll  apparatus  was  discovered 
some  years  ago  by  Engelmann.  Certain  bacteria  which 
contain  a  purple  pigment  were  found  to  possess  the  power 
of  photosynthesis.  The  pigment  was  thought  to  be  allied 
to  chlorophyll  and  to  possess  the  same  power  of  absorbing 
and  utilising  the  radiant  energy  of  light.  Recent  researches 

11  * 


164  VEGETABLE  PHYSIOLOGY 

make  it  probable  that,  like  the  red  seaweeds,  these  organisms 
contain  a  certain  amount  of  chlorophyll,  together  with 
the  purple  pigment. 

Saprophytic  and  parasitic  fungi,  which  contain  no 
chlorophyll,  have  no  power  of  photosynthesis.  They  are 
compelled  to  absorb  their  carbohydrates  from  the  medium 
in  which  they  grow,  and  they  take  them  in  chiefly  in  the 
form  of  sugar.  Parasitic  phanerogams  depend  upon  a 
similar  source  of  supply. 


165 


CHAPTEB  XI 

THE    CONSTRUCTION    OF    PKOTEINS 

The  simple  compounds  containing  nitrogen  which  we  have 
seen  to  be  absorbed  by  the  roots  of  green  plants,  are  as 
unavailable  for  direct  nutrition  as  the  carbon  dioxide  taken 
in  from  the  air.  The  nitrogenous  organic  material  which 
is  actually  assimilated  by  the  protoplasts  is  thought  to  be 
some  form  of  protein.  With  very  few  exceptions,  and  these 
occurring  only  among  micro-organisms,  gelatin  and  similar 
bodies  cannot  be  made  to  support  vegetable  living  sub- 
stance, though  they  can  be  made  use  of  by  animals  to 
supplement,  but  not  to  replace,  their  protein  supplies. 

In  studying  the  story  of  the  construction  of  proteins 
from  the  nitrates  and  ammonia-compounds  taken  into  the 
plant,  we  meet  with  even  greater  difficulties  than  those  which 
are  presented  by  the  photosynthesis  of  carbohydrates.  These 
difficulties  are  connected  with  the  stages  which  occur  in 
the  course  of  the  construction,  with  the  mechanism  which 
is  concerned  in  the  transformation,  and  with  the  condi- 
tions under  which  the  building  up  of  protein  takes  place. 

At  the  outset  of  the  study  we  find  ourselves  in  almost  com- 
plete ignorance  as  to  the  chemical  nature  of  protein.  We 
know  that  it  is  the  most  complex  material  found  in  the  plant 
with  the  exception  of  the  living  substance  itself,  but  we 
know  hardly  anything  about  its  molecular  structure  or  the 
arrangement  or  grouping  of  its  constituent  atoms.  De- 
structive analysis  has  revealed  its  percentage  composition 
within  certain  limits,  although,  as  there  are  many  kinds  of 


166  VEGETABLE  PHYSIOLOGY 

protein  and  all  of  them  are  extremely  difficult  to  prepare  in 
a  pure  condition,  too  much  stress  must  not  be  laid  upon 
the  results  obtained.  These,  moreover,  are,  as  we  should 
expect,  not  altogether  concordant. 

Analysis  of  a  crystallised  protein  prepared  from  the 
seed  of  the  hemp  showed  it  to  have  the  following  percentage 
composition,  which  may  be  taken,  within  somewhat  wide 
limits  to  be  fairly  typical  of  all  : — 

Carbon  .         .         .         .51-58 

Hydrogen  ....         6-88 

Nitrogen  ....       18-8 

Oxygen  ....       21-65 

Sulphur  .         .         .         .         1-09 

Besides  containing  these  essential  constituents,  many 
proteins  leave  on  ignition  a  certain  amount  of  ash.  This 
consists  of  small  amounts  of  the  chlorides,  phosphates,  sul- 
phates, and  carbonates  of  sodium  and  potassium,  with  traces 
of  the  corresponding  salts  of  calcium,  magnesium,  and  iron. 
It  is  not  certain  that  these  ash  constituents  are  an  integral 
part  of  the  protein  molecule  in  any  case  ;  the  balance  of 
evidence  points  rather  to  their  being  impurities  which  are 
very  difficult  of  removal. 

Most  of  the  proteins  found  in  plants  exist  in  an  amor- 
phous condition,  and  are  very  closely  incorporated  with 
the  protoplasm.  In  a  few  cases  they  are  met  with  as 
definite  grains,  and  in  certain  reservoirs  of  food  material 
they  occur  as  crystals.  Some  of  them  can  be  made  to 
crystallise  after  extraction  from  the  organism,  but  many 
forms  exist  which  do  not  possess  this  property,  so  far  as 
we  know  at  present.  It  is  not  certain,  however,  that  the 
crystals  are  always  composed  of  pure  protein  only. 

The  proteins  vary  very  much  among  themselves  as  to 
their  solubility  in  water  and  other  neutral  fluids.  Some 
are  soluble,  others  insoluble,  in  water  ;  some  are  soluble  only 
in  solutions  of  neutral  salts  of  various  degrees  of  concentra- 


THE  CONSTEUCTION  OF  PEOTEINS         167 

tion.  Nearly  all  are  insoluble  in  alcohol  and  ether ;  they 
all  dissolve  in  strong  mineral  acids  and  in  caustic  alkalies, 
but  they  are  decomposed  during  the  process.  Their 
solutions  have  generally  a  power  of  deflecting  a  ray  of 
polarised  light  to  the  left. 

The  best  known  groups  into  which  the  simpler  proteins 
have  been  divided  are  the  following  : — 

1.  ALBUMINS. — These  are  soluble  in  distilled  water,  and 
if  the  solution  is  heated,  the  protein  is  converted  into  a 
peculiarly  insoluble  form,  known  as  coagulated  protein,  and 
deposited  as  a  granular  or  flocculent  precipitate.     As  the 
temperature  rises  the  liquid  becomes  markedly  opalescent 
before  the  separation  of  the  protein.     The  change  takes 
place  at  a  point  which  lies  between  65°  and  80°  C.,  its 
exact  place  depending  upon  the  nature  of  the  albumin  and 
the  reaction  and  concentration  of  the  protein  solution.     This 
point  is  known  as  the  coagulation  temperature.     Albumins 
can  be  precipitated  unchanged  by  saturating  their  solutions 
with  sodio-magnesium  sulphate.     They  are  not  of  frequent 
occurrence  in  plants,  but  can  be  extracted  from  certain 
roots. 

2.  GLOBULINS. — These     differ    from    albumins    in    not 
being  soluble  in  distilled  water.     They  can  be  dissolved  by 
adding  a  little  neutral  salt,  such  as  sodium  chloride.     Their 
solutions  are  coagulated  on  heating,  but  they  show  a  con- 
siderable  variability   as   to   the   coagulation   temperature, 
which  in  the  case  of  some  is  as  low  as  55°  C.     Most  of 
them,  however,  remain  unchanged  below  75°-80°  C.     They 
can   be    precipitated    by    saturating   their    solutions    with 
magnesium  sulphate.     If  sodium  chloride  is  used  instead 
of  the  latter,   an  incomplete   precipitation  usually  takes 
place.     Different    members    of   the    group    show    different 
degrees  of  solubility  in  solutions  of  sodium  chloride  ;   some 
require  only  a  trace  of  the  salt ;  others  need  8-10  per  cent. ; 
and  a  few  are  soluble  only  in  saturated  solutions. 

The   proteins   found  [in   plants    belong    chiefly    to    this 
class.     Globulins  can  be  readily  extracted  from  most  seeds, 


168  VEGETABLE  PHYSIOLOGY 

and  probably  this  form  of  protein  is  the  one  which  occurs 
in  the  green  parts  of  plants. 

Several  albumins  and  globulins  can  be  prepared  in 
crystalline  form. 

3.  META-PROTEINS. — These    are    insoluble    in     distilled 
water  or  in  solutions  of  neutral  salts.     They  are  readily 
soluble  in  very  dilute  acids  and  alkalies,  and  their  solutions 
do  not  coagulate  on  boiling.     They  are  precipitated  by  care- 
fully neutralising  their  solutions,  and  when  they  are  boiled 
in  the  resulting  state  of  suspension  they  are  converted  into 
coagulated  protein,  and  will  not  re-dissolve  on  the  addition 
of  either  dilute  acid  or  alkali. 

They  are  readily  prepared  from  either  albumins  or 
globulins  by  warming  them  in  the  presence  of  a  little  acid 
or  alkali,  preferably  at  about  60°  C.  Alkali-albumin  may 
be  prepared  by  acting  on  albumin  with  fairly  strong  caustic 
potash  in  the  cold. 

The  meta-proteins  are  not  of  frequent  occurrence  in 
plants,  but  may  be  met  with  in  certain  seeds. 

4.  PROTEOSES    or    ALBUMOSES.  —  These    are    generally 
soluble  in  distilled  water,  though  some  are  less  so  than 
others.     They  can  be  precipitated  from  their  solutions  by 
saturating   the   latter   with   neutral   ammonium   sulphate. 
They  differ  from  the  members  of  the  first  two  classes  by 
not  being  converted  into  coagulated   protein  on  boiling. 
Their  characteristic  reaction  is  that  they  give  with  nitric 
acid,  or  with  potassium  ferrocyanide  in  the  presence  of  acetic 
acid,  a  precipitate  which  dissolves  on  warming  the  liquid 
and  reappears  as  it  cools.     Unlike  any  of  those  of  the 
preceding   groups,    they   have   the   property   of    dialysing 
through  a  parchment  membrane,  but  only  very  slowly. 

5.  PEPTONES. — These  are  much  like  albumoses,  but  do 
not  give  a  precipitate  with  nitric  acid  or  with  potassium 
ferrocyanide  in  the  presence  of  acetic  acid.     They  are  not 
precipitated  by  saturation  of  their  solutions  with  ammo- 
nium sulphate,  nor  are  they  coagulated  on  boiling.     Their 
power  of  dialysis  is  much  greater  than  is  that  of  the  proteoses. 


THE  CONSTRUCTION  OF  PKOTEINS         169 

Neither  peptones  nor  proteoses  occur  very  plentifully 
in  plants,  and  they  are  probably  formed  in  them  only  from 
the  decomposition  of  the  more  stable  forms  of  globulin  and 
albumin.  There  is  no  evidence  at  present  that  they  are 
stages  in  the  constructive  process. 

Some  of  the  proteoses  occur  in  certain  seeds  in  associa- 
tion with  some  of  the  globulins.  Both  the  albumins  and 
the  globulins,  and  probably  the  meta-proteins  as  well,  are 
transformed  into  proteoses  and  peptones  by  the  action  of 
hydrolysing  agents,  such  as  dilute  mineral  acids  and  certain 
secretions  of  the  protoplasm  known  as  enzymes,  whose 
action  will  be  treated  of  in  a  subsequent  chapter. 

Besides  these  classes  of  proteins,  another  occurs,  the 
members  of  which  present  the  curious  peculiarity  of  being 
soluble  in  alcohol.  Proteins  of  this  group  have  been  extracted 
from  the  endosperm  of  some  of  the  cereal  grasses.  Examples 
of  them  are  found  in  the  zein  of  maize,  and  the  gliadin  and 
glutenin  of  wheat  flour.  Zein  differs  from  the  crystallised 
protein  of  the  hemp  in  its  comparatively  low  content  of 
nitrogen,  which  amounts  to  only  16'13  per  cent.-  It  dissolves 
easily  in  alcohol  of  about  -820  specific  gravity,  but  is 
insoluble  in  absolute  alcohol.  It  is  insoluble  also  in  water, 
but  in  mixtures  of  alcohol  and  water  it  dissolves  to  a  greater 
or  less  extent,  being  most  easily  soluble  in  a  mixture  con- 
taining about  90  per  cent,  of  the  spirit.  In  one  of  a  lower 
concentration  than  50  per  cent,  it  is  very  sparingly  soluble. 
Zein  can  be  dissolved  by  glycerine  if  heated  to  150°  C. ; 
also  by  glacial  acetic  acid  and  by  dilute  solutions  of  caustic 
potash.  Like  other  proteins,  it  is  converted  into  peptone 
by  peptase  and  hydrochloric  acid. 

The  original  construction  of  protein  matter,  like  that 
of  carbohydrates,  seems  to  be  carried  out  only  by 
vegetable  protoplasm.  It  does  not,  however,  appear  to  be 
dependent  upon  the  same  conditions  as  the  process  already 
described.  It  cannot  be  classed  with  the  latter  as  a  process 
of  photosynthesis,  and  it  is  only  indirectly  dependent 
upon  the  action  of  the  chlorophyll  apparatus.  Unlike  the 


170  VEGETABLE  PHYSIOLOGY 

construction  of  carbohydrates,  it  is  not  confined  to  green 
plants — indeed  the  fungi  can  commence  the  synthesis  at  a 
lower  stage  than  the  latter,  beginning  the  construction  with 
compounds  of  ammonia,  which  have  to  be  converted  into 
nitrates  before  green  plants  can  utilise  them. 

For  the  synthesis  of  proteins  we  have  accordingly  two 
certain  starting-points,  to  which  may  be  added  another 
which  is  confined  to  a  small  group  of  plants,  if  not  indeed 
to  a  single  organism.  We  have  already  alluded  to  the  fact 
that  certain  plants,  chiefly  belonging  to  the  LeguminoseB, 
have  the  power  of  using  the  nitrogen  of  the  atmosphere  for 
the  purpose  of  constructing  organic  food.  This  utilisation 
of  it  is,  however,  not  carried  out  by  the  green  plant  inde- 
pendently, but  only  when  its  roots  are  associated  symbioti- 
cally  with  a  micro-organism  which  usually  forms  peculiar 
tubercular  outgrowths  upon  the  root-branches.  It  is 
apparently  the  micro-organism  which  effects  the  first  fixa- 
tion of  the  nitrogen.  The  leguminous  plant  alone  is  as 
powerless  in  this  direction  as  any  other  green  plant.  How 
the  fixation  takes  place,  what  part  of  it  is  due  to  the  direct 
metabolism  of  the  micro-organism,  and  how  far  the  proto- 
plasm of  the  green  plant  is  concerned  in  the  early  stages, 
are  at  present  quite  uncertain.  It  seems,  however,  probable 
that  the  fixation  is  carried  out  by  the  micro-organism  alone, 
without  any  influence  or  aid  derived  from  the  green  plant. 
A  few  other  similar  organisms  can  under  appropriate 
conditions  carry  on  a  similar  fixation  in  the  soil  without 
being  in  symbiotic  union  with  any  green  plant.  If  this 
view  is  correct,  the  leguminous  plant  is  supplied  by  the 
micro-organism  with  a  food  material  which  has  already 
been  worked  up  from  the  simple  form  in  which  the  elements  of 
it  are  absorbed ;  but  how  far  the  manufacture  has  proceeded 
— that  is  to  say,  in  what  condition  the  nitrogenous  material 
is  actually  presented  to,  and  absorbed  by,  the  tissues  of 
the  root — is  at  present  uncertain. 

The  power  of  fixation  of  free  nitrogen  thus  possessed 
by  the  organisms  mentioned  has  been  stated  by  several 


THE  CONSTKUCTION  OF  PKOTEINS         171 

observers  to  be  shared  by  certain  lowly  Algae,  but  the 
evidence  as  to  their  activity  in  this  direction  is  conflicting. 
It  may  be  that  they  are  capable  of  a  similar  symbiotic 
relationship  with  certain  of  the  nitrogen-fixing  bacteria  of 
the  soil  already  mentioned,  but  it  is  more  probable  that  it 
is  carried  out  by  bacteria  living  simultaneously,  but  not 
symbiotically,  in  the  soil  with  them.  Several  of  these 
organisms  appear  to  be  associated  symbiotically  with  each 
other,  though  not  with  any  green  plant. 

When  we  turn  to  the  method  of  construction  of  protein 
by  a  green  plant  we  find  ourselves  in  possession  of  very 
little  accurate  information  as  to  the  stages  which  are 
involved.  We  find  that  nitrates  especially  are  absorbed 
by  the  root-hairs  from  the  soil,  and  that  a  continuous 
stream  of  them  passes  into  the  plants.  This  naturally 
is  associated  with  a  transportation  of  the  nitrates  through 
the  root  and  stem.  They  can  be  detected  in  varying 
quantities  in  these  regions,  but  the  amount  seems  to  diminish 
as  the  termination  of  the  stem  is  approached,  and  little 
can  be  found  to  be  present  in  the  leaves.  It  may  be  inferred 
that  a  gradual  decomposition  takes  place  as  they  pass 
along  the  axis,  and  that  this  is  completed  in  the  leaves. 

A  theory  has  been  advanced  to  explain  this  disappear- 
ance, which  may  be  mentioned  here.  It  is  that  the  nitrates 
are  decomposed  by  the  organic  acids  of  the  plant,  and  in 
particular  by  oxalic  acid.  Simultaneously  the  sulphates 
which  are  absorbed  undergo  a  similar  fate.  The  resulting 
bodies,  the  nitric  and  sulphuric  acids,  unite  with  some  form 
of  non-nitrogenous  organic  substance,  possibly  formal- 
dehyde, or  a  fairly  simple  carbohydrate,  to  form  protein. 
From  what  has  already  been  advanced,  however,  it  is  evident 
that  this  scheme  of  construction  is  purely  hypothetical. 

When  we  search  for  a  form  of  nitrogen  compound  that 
is  nearer  protein  in  its  composition  than  these  simple  salts, 
it  is  natural  to  look  at  the  products  of  the  decomposition  of 
protein  material  to  see  if  these  furnish  any  clue  to  a  possible 
constructive  process.  When  proteins  are  digested  in  the 


172  VEGETABLE  PHYSIOLOGY 

animal  organism  under  the  influence  of  the  strong  tryptase 
of  the  pancreatic  secretion,  we  find  that  among  the  products 
of    the     decomposition    certain    nitrogenous     compounds 
occur  which  are  crystalline  and  capable  of  diffusing  through 
animal  and  vegetable  membranes.     These  substances,  the 
chief  of  which  are  tyrosin  and  leucin,  with  a  little  asparagin, 
are  known  technically  as  amino-  and  amido-acids,  owing  to 
their  containing  the  group  NH3  (amidogen),  replacing  an 
atom  of  hydrogen  in  some  portion  of  the  grouping  of  the 
atoms  in  an  organic  acid.     It  is  extremely  probable  that 
these  compounds  are  made  use  of  again  in  the  subsequent 
reconstruction   of   proteins   in   the   cells.     Many   of   these 
substances  have  been  found  to  occur  in  plants,  and  among 
them    asparagin    is    extremely    conspicuous.     It    can    be 
detected  in  seeds  and  seedlings,  and  in  older  plants  it  is 
not  infrequently  present  in  the  leaves.     There  is  consider- 
able probability  that  these  substances  occur  as  a  stage  in 
the  original  construction  of  proteins,  though  they  may  no 
doubt  also  be  formed  during  its  digestion  in  the  vegetable 
as  well  as  in  the  animal  organism.     This  probability  is 
supported  by  the  observation  that  green  plants  are  able  to 
absorb  from  the  soil  and  utilise  many  such  complex  acids 
when  artificially  supplied  to  them. 

Another  hypothesis  of  protein  construction  has  been 
advanced  which  takes  account  of  these  substances  as  stages 
in  the  process.  We  have  seen  that  salts  of  ammonia  are 
converted  into  nitrates  in  the  soil  before  being  absorbed. 
The  first  step  in  the  construction  is  thought  to  be  the  re- 
conversion of  the  nitrates  into  ammonia,  which  interacts  in 
some  way  with  formaldehyde  or  one  of  its  polymerides  to  form 
one  or  other  of  these  complex  acids.  This  subsequently 
combines  with  some  kind  of  non-nitrogenous,  organic  sub- 
stance, together  with  some  compound  of  sulphur,  to  form 
protein. 

A  theory  of  the  method  of  the  first  formation  of  amido- 
compounds  was  put  forward  by  Bach  in  1896.  He  suggested 
that  the  absorbed  nitrates  are  decomposed  by  the  organic 
acids  of  the  plant,  and  that  the  liberated  nitric  acid  is 


THE  CONSTKUCTION  OF  PROTEINS         173 

reduced  by  the  formaldehyde,  part  of  which  combines  with 
the  resulting  product  to  form  hydroxylamine  and  later 
formaldoxine,  which  is  then  converted  into  formamide. 

The  view  of  the  construction  of  protein  from  amido- 
compounds  and  carbohydrates,  though  of  course  only 
hypothetical,  associates  certain  processes  which  apparently 
occur  in  nature.  Its  formation  seems  to  involve  the  simul- 
taneous presence  in  the  cells  of  some  amino-  or  amido-acid, 
frequently  asparagin,  and  some  carbohydrate  such  as  sugar. 
If  shoots  of  plants  which  exhibit  no  accumulation  of  asparagin 
during  normal  growth  are  cut  off  and  kept  in  darkness  for 
some  time,  a  gradual  accumulation  of  the  amido-acid  can 
be  observed.  This  in  all  probability  is  the  expression  of 
the  decomposition  of  protein  taking  place  during  the  life 
of  the  shoot,  and  is  presumably  a  normal  occurrence.  The 
reconstruction  which  would  explain  its  non-accumulation 
during  illumination  is  prevented  by  the  non-formation  of 
the  needed  carbohydrate  in  the  darkness. 

The  probability  of  a  combination  or  interaction  of  these 
two  classes  of  substance  in  the  synthesis  of  proteins  is 
supported  by  the  fact  that  at  the  active  growing  points, 
where  protoplasm  is  energetically  formed,  and  where  con- 
sequently abundant  supplies  of  proteins  are  needed,  neither 
sugar  nor  amido-acids  can  be  detected,  though  they  can 
be  traced  quite  readily  up  to  a  short  distance  below  the 
place  where  this  active  growth  is  proceeding.  This  fact  is 
easily  understood  if  we  admit  that  protein  is  constructed 
there  at  the  expense  of  these  two  constituents,  supplemented, 
of  course,  by  the  necessary  compound  or  compounds  of 
sulphur.  If  either  of  these  supplies  ceases  to  be  available, 
the  growth  of  the  plant  at  that  point  stops. 

Though  we  have  seen  reasons  for  thinking  that  nitrates 
and  amido-acids  form  two  stages  in  the  normal  process  of 
protein  construction,  we  must  not  conclude  that  they  in- 
variably do  so.  In  one  plant,  Pangium  edule,  which  was 
examined  by  Treub  in  1894,  the  nitrogen  needed  for  protein 
construction  appears  to  be  supplied  in  the  form  of  hydro- 
cyanic acid.  In  the  shoots  of  this  plant,  cells  occur  in  the 


174  VEGETABLE  PHYSIOLOGY 

cortex  which  contain  this  acid.  In  those  nearest  the  apex 
the  latter  occurs  alone,  but  as  they  grow  older,  a  little 
protein  is  found  to  be  mixed  with  it.  In  still  older  ones 
the  protein  preponderates,  and  at  some  distance  behind  the 
seat  of  growth  it  occurs  alone,  the  acid  having  disappeared. 
Certain  fungi  can  utilise  nitrogen-containing  derivatives  of 
methane  or  benzol  for  the  same  purpose.  It  is  possible, 
therefore,  that  more  than  one  pathway  to  the  protein  mole- 
cule may  yet  be  found  in  different  plants. 

Probably  the  construction  of  protein  is  not  confined  to 
any  definite  tissue  or  series  of  tissues  in  the  plant.  It  is 
certainly  only  connected  indirectly  with  the  chlorophyll 
apparatus,  and  that  in  so  far  as  carbohydrates  are  necessary 
for  its  formation.  At  the  same  time,  there  is  a  certain 
amount  of  evidence  which  points  to  its  synthesis  being  in  the 
first  place  effected  in  the  leaves.  The  fact  that  nitrates 
can  be  traced  towards  these  organs,  and  that  they  never- 
theless do  not  appear  to  be  present  in  the  mesophyll  cells, 
makes  it  probable  that  they  are  manufactured  into  some- 
thing else  there.  The  occurrence  of  amido-acids  in  the 
leaves  is  more  in  harmony  with  the  view  that  they  are  built 
up  there  than  with  the  assumption  that  they  arise  from  the 
decomposition  of  already  existing  proteins,  though,  no 
doubt,  the  latter  is  the  case  in  the  tissues  of  seeds,  and 
possibly  of  seedlings,  which  are  being  nourished  at  the 
expense  of  materials  stored  in  the  seed.  The  proportion 
of  protein  to  dry  weight  of  tissue  has  been  stated  to  increase 
gradually  and  progressively  from  the  roots  to  the  leaves,  in 
which  it  attains  a  maximum.  Moreover,  proteins  are 
continually  being  removed  from  the  leaves.  If,  however, 
the  process  does  primarily  go  on  in  the  leaves,  it  does  not 
take  place  under  the  same  conditions  as  the  construction  of 
carbohydrates.  It  goes  on  quite  well  in  green  cells  in 
darkness,  so  that  it  is  not,  as  already  mentioned,  a  process 
of  photosynthesis.  It  has  recently  been  claimed  that  the 
construction  of  protein  in  certain  plants  is  favoured  by 
light,  and  more  particularly  by  the  ultra-violet  rays,  though 


THE  CONSTEUCTION   OF  PEOTEINS          175 

the  luminous  ones  have  a  certain  feeble  effect.  Whether 
or  no  the  energy  for  the  construction  is  derived  therefrom 
is  not,  however,  certain. 

Sachs  held  that  the  sieve-tubes  of  the  nbro-vascular 
bundles  of  the  axis  of  the  plant  are  also  the  seat  of  the 
construction  of  protein.  Though  this  is  possible,  it  seems 
more  likely  that  they  are  concerned  in  the  transmission 
of  organic  nitrogenous  material  from  the  leaves  to  other 
organs.  In  whatever  form  protein  material  travels  about 
the  plant,  which  for  the  present  we  cannot  discuss,  it  is 
almost  certain  that  it  passes  by  the  sieve-tubes,  and  it  may 
well  be  that  too  great  an  accumulation  of  the  travelling 
form  may  be  attended  by  its  conversion  into  an  insoluble 
condition,  and  its  deposition  in  the  cells.  There  is  no 
conclusive  evidence  pointing  to  the  sieve-tubes  as  the  places 
where  it  is  originally  synthesised. 

The  same  considerations  apply  to  the  various  growing 
points  or  zones.  There  is  little  doubt  that  protein  is  con- 
structed there,  but  it  is  probable  that  it  is  so  built  up  from 
bodies  which  have  resulted  from  the  digestion  or  decom- 
position of  protein  that  has  already  been  synthesised  else- 
where, and  which  has  undergone  such  decomposition  solely 
with  a  view  to  transport  or  translocation. 

We  judge  it  probable  on  all  these  grounds  that  the  great 
seat  of  protein  construction  in  a  green  plant  is  the  leaves, 
and  this  not  on  account  of  the  possession  of  the  chlorophyll 
apparatus,  but  because  of  a  property  inherent  in  the  cell- 
protoplasm.  Whence  the  energy  is  derived  is  not  clear,  but 
many  writers  hold  it  to  be  supplied  by  accompanying 
chemical  decompositions. 

The  construction  of  protein  by  fungi  is  an  additional  proof 
that  it  is  altogether  independent  of  the  chlorophyll  appa- 
ratus, if  not  that  it  is  unconnected  with  the  access  of  light. 

The  third  group  of  foods,  the  fats  or  oils,  are  probably 
not  directly  synthesised  in  plants,  but  are  products  of  the 
decomposition  of  proteins,  or  perhaps  of  the  living  sub- 
stance itself. 


176  VEGETABLE  PHYSIOLOGY 


CHAPTEK  XII 

THE    CONSTITUENTS   OF   THE   ASH   OF   PLANTS 

We  have  seen  in  a  previous  chapter  that  when  a  plant  is 
carefully  burned  and  the  residue  collected,  the  latter,  which 
is  known  as  the  ash,  is  found  to  contain  a  number  of  elements 
which  vary  in  different  cases  and  which  always  include 
certain  metals,  as  well  as  some  non-metallic  elements.  The 
occurrence  of  this  ash  being  universal,  we  can  conclude 
without  any  difficulty  that  some  of  its  constituents  at  least 
must  be  of  importance  to  the  organism,  though  it  cannot 
be  denied  that  our  information  is  exceedingly  incomplete. 
It  is  certainly  possible,  if  not  actually  probable,  that  a 
definite  association  of  any  of  the  elements  with  a  particular 
function  does  not  and  cannot  exist.  Even  in  the  animal 
body,  the  study  of  which  is  far  more  complete  than  that 
of  the  plant,  such  association  has  not  been  found.  Each 
element  plays  more  than  one  part,  and  not  improbably  the 
role  it  plays  at  any  moment  depends  to  a  very  large  extent 
on  the  condition  of  the  organism.  We  may  well  conclude 
that  in  the  organisation  of  the  plant  also  there  is  no  definite 
devolution  of  a  particular  function  to  a  single  constituent 
of  its  composition.  It  is  probable  that  the  well-being  of 
any  organism  depends  on  the  interaction  of  many  elements 
with  the  protoplasm — interaction  which  may  vary  from 
time  to  time  and  from  place  to  place,  according  to  the 
changes  of  the  environment  or  the  automatic  readjustments 
going  on  in  the  living  substance.  The  influence  of  varying 
quantities  of  the  mixture  of  elements  may  be  considerable. 
Correlations  of  the  functions  of  the  plant,  or  the  abnormal 


THE  CONSTITUENTS  OF  THE  ASH  OF  PLANTS     177 

performance  of  one  or  more  of  them  under  the  influence  of 
disturbance  of  quantitative  relations  may  obscure  the  action 
of  any  element,  or  group  of  elements,  in  whatever  combina- 
tion it  or  they  may  be  existing.  Probably  all  the  essential 
elements  of  the  ash  play  many  parts,  all  co-ordinated  by 
the  living  substance  of  the  organism.  Whether  any  of  the 
constituents  of  the  ash  actually  enter  into  the  composition 
of  protoplasm  is  doubtful,  but  several  of  them  appear  to 
be  necessary  for  the  assimilation  of  the  food  which  is  either 
manufactured  or  supplied,  as  well  in  the  case  of  the  vegetable 
as  in  that  of  the  animal  organism. 

From  the  nature  of  the  plant-body  and  the  absence  of  the 
localisation  of  different  functions  in  particular  organs  which 
is  so  much  more  clearly  characteristic  of  the  animal  organism, 
it  becomes  very  difficult  to  ascertain  the  exact  nature  of  any 
part  played  by  any  of  these  ash  constituents.  We  can  more 
easily  determine  what  is  the  effect  produced  by  variations  in 
the  amount  supplied  or  by  the  total  absence  of  any  of  them. 
This  effect  is  usually,  however,  only  the  general  effect  upon 
the  plant,  and  the  experiments  leave  us  still  quite  in  the 
dark  as  to  the  way  in  which  any  general  effect  is  produced, 
whether  directly  or  indirectly  by  affecting  the  health  of  the 
plant  and  thus  leading  to  secondary  changes  in  its  tissues. 

The  experiments  in  question  are  preferably  carried  out 
by  means  of  water-culture,  the  general  nature  of  which  wet 
have  already  explained.  Plants  will  grow  very  well  in 
water  containing  small  quantities  of  various  inorganic  salts, 
and  these  can  be  varied  at  will  for  the  purpose  of  definite 
inquiries.  The  composition  of  such  a  culture-solution  is 
given  by  Pfeffer  as  under  : — 

Calcium  nitrate         .         .         4     grms. 

Potassium  nitrate      .         .         1      grm. 

Magnesium  sulphate  .         1      grm. 

Potassium  acid  phosphate  .         1      grm. 

Potassium  chloride    .         .         0-5  grm. 

Ferric  chloride  solution      .         a  few  drops 

Water  ...         7     litres 

12 


178  VEGETABLE  PHYSIOLOGY 

Or  a  convenient  fluid  may  be  prepared  by  dissolving 
2O5  grms.  magnesium  sulphate  in  350  c.c.  of  water,  and 
40  grms.  calcium  nitrate,  10  grms.  potassium  nitrate,  and 
10  grms.  acid  phosphate  of  potassium  in  another  350  c.c.  ; 
100  c.c.  of  each  of  these  solutions  should  then  be  added  to 
9 -8  litres  of  water.  This  culture-medium  will  contain  0-2 
per  cent,  of  salts,  and  will  need  only  the  further  addition 
of  a  few  drops  of  ferric  chloride  solution. 

This  percentage  is  generally  satisfactory,  though  'the 
concentration  may  be  increased  twofold  without  affecting 
the  plants  injuriously.  Too  great  a  quantity  of  salts, 
however,  becomes  deleterious. 

The  effect  of  omitting  any  particular  constituent  can 
be  examined  by  making  up  the  culture-fluid  as  required. 
Fig.  89  shows  the  effect  of  varying  it  in  certain  particulars. 
Pot  1  contains  such  a  fluid  as  is  described  above  ;  in  pot  2 
is  no  potassium  :  in  pot  3  potassium  is  replaced  by  sodium  ; 
in  pot  4  is  no  calcium,  while  from  pot  5  all  compounds  of 
nitrogen  are  absent.  The  general  character  of  such  experi- 
ments can  be  seen  by  comparing  the  relative  development  of 
the  plants  under  these  conditions,  and  it  is  at  once  evident 
that  the  different  metals  and  other  elements  employed  have  a 
certain  functional  importance.  Deprivation  of  any  of  those 
mentioned  affects  all  plants  injuriously,  though  in  different 
degrees. 

We  can,  however,  say  very  little  as  to  the  way  in  which 
the  injurious  effects  are  produced  in  different  cases.  We 
can,  as  a  rule,  only  guess  at  the  functions  of  the  different 
ash  constituents  by  studying  the  effects  thus  made  evident. 
In  a  very  few  cases  we  can  associate  an  element  with  some 
definite  metabolic  process.  An  instance  is  afforded  by  the 
behaviour  of  iron,  in  the  absence  of  which,  as  we  have 
seen,  there  is  no  development  of  chlorophyll  in  the  chloro- 
plasts.  We  cannot  even  here  say  very  definitely  how  this 
inhibition  is  caused.  It  seems  unlikely  that  it  is  directly 
concerned  with  the  manufacture  of  chlorophyll,  for  all 
analyses  of  the  latter  show  that  iron  does  not  enter  into 


THE  CONSTITUENTS  OF  THE  ASH  OP  PLANTS      179 

its  molecule.     It  is  probably  an  indirect  effect  arrived  at 
through  faulty  nutrition  set  up  in  the  absence  of  the  metal. 


4  3125 

FIG.  89. — PLANTS  OF  BUCKWHEAT  CULTIVATED  IN  VABIOUS  NUTRITIVE 
SOLUTIONS. 

1,  normal  solution  containing  all  necessary  salts;  2,  solution  containing  the  same 
salts  as  1,  except  potassium  compounds ;  3,  solution  of  same  composition  as  1, 
except  that  sodium  salts  have  been  substituted  for  potassium  compounds  ;  4, 
solution  of  same  composition  as  1,  except  that  no  calcium  salts  are  present; 
5,  solution  containing  no  compounds  of  nitrogen. 


At  first  sight  it  seems  as  if  the  absence  of  inorganic  salts 
may  be  effective  by  interfering  with  the  maintenance  of  the 
turgid  condition  of  the  cells,  as  all  the  compounds  men- 
tioned have  osmotic  properties.  It  is  evident,  however, 

12* 


180  VEGETABLE  PHYSIOLOGY 

that  this  cannot  be  the  only  or  even  the  main  cause  of  the 
disturbance  of  nutrition,  as  the  salts  are  not  interchange- 
able, and  a  salt  of  sodium  in  concentration  quite  sufficient 
to  maintain  the  condition  of  turgor  is  unable  to  replace 
the  salts  of  potassium  normally  required.  Moreover,  tur- 
gescence  can  be  maintained  by  organic  acids  in  the  total 
absence  of  the  normal  constituents  of  the  ash. 

We  can  divide  the  latter  into  four  groups  which  subserve 
different  purposes.  Of  these  the  members  of  the  first  are 
essential,  because  they  enter  into  the  constitution  of  the 
living  substance.  They  are  sulphur  and  'phosphorus.  All 
analyses  of  proteins  show  that  sulphur  is  an  essential  con- 
stituent of  them,  and  as  proteins  are  immediately  applied 
to  the  construction  of  protoplasm,  there  can  hardly  be  any 
doubt  that  sulphur  is  contained  in  living  substance.  Phos- 
phorus does  not  seem  to  be  present  in  the  ordinary  cyto- 
plasm, but  is  undoubtedly  associated  with  the  nucleus. 
The  nature  of  the  connection  is  not  very  clear,  but  all  nuclei 
contain  a  constituent  which  bears  the  name  of  nuclein. 
This  can  be  extracted  from  it  by  appropriate  treatment, 
and  analysis  shows  that  phosphorus  enters  into  its  molecule. 
Nuclein  occurs  also  in  the  substance  of  many  cells,  either 
as  nucleic  acid,  or  associated  with  certain  protein  bodies. 

The  second  group  comprises  certain  metals  which  are 
essential  to  the  development  of  a  plant,  but  which  appa- 
rently do  not  ever  form  part  of  the  living  substance. 
There  is  some  little  doubt  about  this,  as  the  fact  cannot 
be  ascertained  by  analysis.  The  members  of  this  group 
are  potassium,  magnesium,  calcium,  and  iron. 

The  third  group  includes  several  elements  which  are 
not  absolutely  essential,  but  which  are  useful  in  many 
cases,  and  which  are  very  widely  distributed,  although  not 
universally  present.  Among  them  are  sodium,  silicon, 
manganese,  chlorine,  bromine,  and  iodine. 

:The  fourth  group  includes  many  other  elements  which 
are  only  occasionally  present,  and  which  probably  play  no 
part  in  the  metabolic  processes.  They  appear  to  be  absorbed 


THE  CONSTITUENTS  OF  THE  ASH  OF  PLANTS     181 

because  they  are  present  in  the  particular  soil  in  which  the 
plant  happens  to  be  growing,  and  have  the  power  of  osmosing 
through  the  cell-membranes  of  the  root-hairs.  Many  of 
them  have  only  been  found  in  a  few  plants.  Among  them 
may  be  mentioned  aluminium,  zinc,  copper,  cobalt,  nickel, 
zirconium,  fluorine,  and  lithium. 

What  is  frequently  spoken  of  as  the  selective  power  of 
plants  is  often  misunderstood.  If  a  substance  is  present 
in  a  soil,  can  be  made  soluble  in  the  hygroscopic  water 
permeating  that  soil,  and  can  dialyse  through  the  living 
membrane  of  the  root -hair,  absorption  ot  a  certain  quantity 
of  it  will  take  place.  How  much  is  ultimately  absorbed  is 
a  question  ot  the  power  of  the  plant  to  decompose  or  utilise 
it  after  absorption.  Many  substances  which  are  useless  or 
even  deleterious  to  the  plant  which  takes  them  up  are 
absorbed  continuously  until  a  very  large  percentage  of 
them  is  present,  because  other  constituents  of  the  plant 
decompose  them,  or  because  their  power  of  dialysis  is  such 
that  they  are  easily  removed  from  the  absorbing  cells. 
The  possibility  of  the  dialysis  by  which  they  are  originally 
taken  up  is  perhaps  a  question  of  relationship  between  the 
size  of  their  molecules  and  that  of  the  meshes  of  the  proto- 
plasmic membranes  which  bound  the  cytoplasm  on  its  two 
faces,  abutting  on  the  cell-wall  and  the  vacuolar  cavity 
respectively.  Or  it  may  be  a  question  of  the  protoplasm 
actually  picking  them  out  of  the  watery  stream  and  passing 
them  into  the  cell  apart  from  osmosis  altogether.  This 
possibility  of  penetrating  into  the  cell,  and  the  power  of 
subsequently  removing  the  substances  therefrom,  are  the 
special  features  of  the  so-called  selective  power  of  the 
plant,  and  it  is  evident  that  this  power  is  particularly  asso- 
ciated with  the  disposition  of  the  materials  after  absorption, 
more  than  with  the  absorption  itself. 

We  may  now  turn  to  the  consideration  of  these  varied 
constituents  of  the  ash,  and  examine  them  in  detail.  The 
first  group,  we  have  seen,  is  composed  of  sulphur  and 
phosphorus.  Its  importance  lies  in  the  fact  that  these 


182  VEGETABLE  PHYSIOLOGY 

elements  enter  into  very  close  relationship  with  protoplasm, 
the  former  at  any  rate  being  in  all  probability  a  constituent 
of  its  molecule. 

Sulphur  is  only  taken  up  by  the  higher  plants  in  the  form 
of  sulphates  of  some  of  the  metals  of  the  other  groups  or 
of  ammonia.  Fungi  can  also  utilise  salts  of  sulphurous  and 
hydrosulphurous  acids  when  they  are  presented  in  dilute 
solutions. 

Phosphorus  is  associated  with  the  nucleus  rather  than 
with  the  cell-protoplasm.  It  is  contained  in  the  substance 
called  nuclein  to  the  extent  of  about  6  per  cent.  The 
nuclein  is  apparently  chiefly  in  the  chromatin  substance 
of  the  nucleus.  Phosphorus  is  also  a  constituent  of  some 
proteins,  and  is  probably  present  in  the  enzymes  which 
are  concerned  in  the  true  digestive  processes  of  the  plant. 
It  occurs  in  chlorophyll  also,  according  to  Hoppe-Seyler, 
whose  analysis  of  this  pigment  has  already  been  quoted 

(page  149).  In  a  few  plants  phos- 
phorus is  temporarily  stored  in  the 
seeds.  Examples  are  presented  by 
the  Brazil  nut  (Bertholletia)  and 
the  Castor-oil  plant  (Ricinus),  whose 
seeds  contain  stores  of  protein 

FIG.  90.— CELL  or  RIOINUS  .   ,    .       ,,        ,  , 

SEED,  CONTAINING  FIVE         material  in  the  form  of  complex 

ALEUBONE  GRAINS.  grains.  In  the  interior  of  these 

grains  there  is  a  small,  usually 

round,  substance  known  as  a  globoid,  consisting  chiefly  of 
a  double  phosphate  of  calcium  and  magnesium  (fig.  90), 
which  lies  side  by  side  with  a  crystal-like  protein  body. 

Lecithin,  a  complex  fatty  body  containing  phosphorus, 
is  present  in  actively  growing  cells  in  many  plants. 

Phosphorus  is  absorbed  by  the  plant  usually,  if  not 
entirely,  in  the  form  of  soluble  phosphate,  most  frequently 
a  phosphate  of  calcium.  Besides  being  important  as  an 
integral  part  of  the  living  substance,  certain  observations 
tend  to  show  that  it  assists  materially  in  the  construction 
of  proteins. 


THE  CONSTITUENTS  OF  THE  ASH  OF  PLANTS     183 

The  second  group  of  ash  constituents  includes  four 
metals  which  are  essential  to  all  plants,  viz.  potassium, 
magnesium,  calcium,  and  iron.  Probably  these  act  only 
indirectly  in  the  constructive  processes,  though  there  is 
some  evidence  that  they  may  be  integral  constituents  of 
living  substance.  They  do  not  enter  into  the  composition 
of  proteins. 

Potassium  is  absorbed  in  a  variety  of  compounds,  of 
which  the  nitrate  and  the  chloride  are  the  most  advan- 
tageous. The  part  which  it  plays  is  not  at  all  well  under- 
stood. It  may  enter  into  the  composition  of  protoplasm, 
for  it  is  especially  abundant  in  embryonic  tissues.  It  has 
been  thought  to  be  connected  with  the  construction  of 
carbohydrates,  but  in  what  way  is  not  known.  It  occurs 
in  greatest  quantity  in  the  organs  in  which  the  formation 
and  storage  of  these  bodies  are  most  actively  carried  out, 
viz.  leaves,  tubers,  seeds,  &c. 

Magnesium  has  a  distribution  much  like  that  of  po'as- 
sium,  and,  as  well  as  calcium,  is  thought  by  some  botanists 
to  enter  into  the  composition  of  protoplasm.  It  may  be 
absorbed  in  various  combinations,  but  the  chloride  is  the 
least  advantageous.  Calcium  is  essential  to  all  green 
plants,  but  fungi  do  not  always  require  it.  Little  of  it 
relatively  is  found  in  young  tissues,  but  greater  amounts 
are  present  in  adult  ones.  Its  function  is  not  understood, 
but  it  is  useful  in  neutralising  oxalic  acid.  It  is  prominent 
in  the  cell-wall,  part  of  which  even  in  the  very  young  state 
consists  of  calcic  pectate.  In  older  cells  the  middle  lamella 
appears  to  consist  entirely  of  this  substance  until  lignification 
is  complete.  Calcium  may  be  absorbed  in  the  same  com- 
binations as  magnesium. 

As  has  been  already  mentioned,  the  most  evident  function 
of  iron  is  to  assist  in  the  formation  of  chlorophyll.  As  it  is 
not  contained  in  the  pigment,  its  influence  here  can  only  be 
indirect.  It  may  be  associated  in  some  way  with  the  proto- 
plasmic basis  of  the  plastid,  so  that  the  latter  in  its  absence 
is  thrown  into  a  pathological  condition  and  ceases  to  form 


184  VEGETABLE  PHYSIOLOGY 

the  colouring  matter.  The  influence  both  of  the  metal  and 
of  light  in  this  particular  may  consequently  be  similar. 
That  it  is  associated  with  the  plastid  does  not  appear  im- 
probable in  view  of  some  observations  of  Macallum's  that 
iron  is  always  found  in  direct  relationship  with  the  chromatin 
of  the  nucleus,  of  which  it  appears  to  be  an  integral  part. 
There  is  here  evidence  of  a  close  association  between  the 
metal  and  some  forms  of  living  substance. 

Iron  can  be  absorbed  with  advantage  apparently  in  any 
soluble  combination. 

The  third  group  of  elements  comprises  several  that  are 
of  importance  to  particular  plants,  but  are  not  universally 
necessary.  Others  usually  included  here  are  not  known 
to  be  functionally  important  at  all,  except  that  they  have 
a  certain  power  of  replacing  to  some  extent  the  more 
important  metals  which  have  been  already  spoken  of. 

Of  the  metals  of  this  group,  sodium  is  the  most  widely 
distributed.  It  exists  in  all  soils,  and  it  is  capable  of 
absorption  in  considerable  quantities.  Experiments  by 
means  of  water-culture  show,  however,  that  its  beneficial 
influence  is  extremely  slight.  It  can  be  omitted  from  the 
culture-fluid  without  entailing  any  harm  to  the  plant,  and 
its  presence  in  any  quantity  will  not  compensate  for  the 
absence  of  potassium  (fig.  89,  1  and  8).  If  compounds  of 
sodium  and  potassium  are  present  together  in  sufficient 
quantity,  the  latter  is  always  absorbed  in  far  the  largest 
amount,  indeed  almost  exclusively  by  many  plants.  So- 
dium seems  able,  however,  to  effect  a  certain  economy 
in  the  use  of  potassium.  If  a  cereal  plant  is  supplied 
with  too  little  potassium,  and  with  a  certain  amount  of 
sodium,  development  is  normal,  and  an  examination  of 
the  distribution  of  the  two  metals  in  its  tissues  shows 
that  the  potassium  is  accumulated  in  the  flowers  and  seeds, 
while  the  sodium  replaces  it  in  the  vegetative  parts.  It  is 
absorbed  in  the  same  combinations  as  potassium,  but  the 
chloride  is  not,  as  in  the  latter  case,  a  valuable  salt. 
Indeed,  sodium  chloride  is  generally  deleterious,  except, 


THE  CONSTITUENTS  OF  THE  ASH  OF  PLANTS     185 

perhaps,  to  the  plants  of  the  sea-shore,  in  which  it  pro- 
motes their  peculiar  succulence. 

If  we  compare  the  influence  of  potassium,  sodium,  and 
calcium  on  the  development  of  a  crop  of  herbage  plants, 
we  find  that  the  presence  of  potassium  leads  to  a  develop- 
ment of  stems,  flowers,  and  fruit,  or  to  what  may  be  regarded 
as  the  maturing  of  the  plants  ;  while  in  the  absence  of 
sufficient  potassium  and  the  presence  of  calcium  and  sodium 
vegetative  growth  is  more  directly  favoured,  but  the  crop 
remains  backward  and  immature. 

There  is  a  possibility  that  all  these  metals  serve  another 
purpose  as  well  as  some  particular  functional  one.  We 
have  seen  that  the  nitrogen  which  the  plant  obtains  is 
derived  from  the  soil,  being  most  favourably  supplied  by 
the  latter  in  the  shape  of  nitrates.  In  the  soil  the  nitric 
acid  is  combined  most  frequently  with  the  metals  under 
discussion,  and  a  not  inconsiderable  quantity  of  the  latter 
may  be  taken  up  solely  for  the  sake  of  the  nitrogen  which 
they  can  thus  carry  into  the  plant.  The  varying  amounts 
of  sodium  and  calcium  which  plants  contain  have  been 
found  to  bear  a  certain  relationship  to  the  amounts  of 
their  compounds,  which  occur  in  the  particular  soils  in 
which  the  plants  have  been  growing.  When  calcium  and 
sodium  nitrates  are  taken  up  for  the  sake  of  the  nitrogen, 
they  are  probably  decomposed  by  the  organic  acids  formed 
in  the  plant,  and  the  nitrogen  is  made  to  enter  into  further 
combination,  leading  to  the  construction,  possibly  of  amino- 
or  amido-acids,  and  eventually  of  proteins. 

Of  the  other  elements  which  are  included  with  sodium 
in  this  group,  silicon  is  one  of  the  most  prominent.  It  is 
absorbed  almost  entirely  in  the  form  of  silicates  of  potas- 
sium and  sodium,  the  latter  combination  being  the  prin- 
cipal one.  It  is  difficult  to  say  what  purpose  it  serves. 
It  is  usually  found  deposited  in  the  epidermal  cell-walls, 
and  as  the  grasses  and  the  horsetails  contain  it  in  greatest 
abundance,  it  has  been  suggested  that  its  utility  consists  in 
its  contributing  to  the  rigidity  of  their  weak  stems,  and 


186  VEGETABLE  PHYSIOLOGY 

consequently  to  the  maintenance  of  their  vertical  position. 
This  is,  however,  not  the  case  ;  their  rigidity  is  dependent 
on  the  degree  of  development  of  their  harder  tissues,  and 
the  absence  of  silica  makes  but  little  difference  to  them. 
Silicates,  when  added  in  quantity  to  the  soil  in  which  green 
crops  are  growing,  have  no  marked  effect  upon  the  amount 
of  silica  which  is  subsequently  present  in  the  straw.  It  is 
uncertain  whether  the  silica  enters  into  the  metabolism  of 
the  plant,  or  whether  the  silicates  are  decomposed  at  once 
and  the  silica  deposited  in  the  cell-walls  in  which  it  is 
prominent.  As  it  is  most  readily  taken  up  in  combination 
with  sodium,  this  is  unlikely,  the  sodium,  being,  as  we  have 
seen,  of  very  little,  if  any,  use.  It  has  been  said  that  oats 
mature  less  fully  and  completely  in  the  absence  of  silica, 
so  that  in  the  case  of  that  particular  plant  there  is  some 
evidence  of  its  aiding  in  metabolism,  though  no  suggestion 
has  been  made  as  to  the  way  in  which  it  exerts  its  influence. 
It  is  possible  that  it  may  be  of  value  also  by  protecting 
the  plant  from  the  depredations  of  animals  or  from  the 
attacks  of  fungi,  as  it  is  mainly  accumulated  in  the 
epidermis. 

The  other  elements  of  this  group  include  chlorine, 
bromine,  and  iodine.  A  little  of  the  former  is  of  universal 
occurrence,  but  it  may  be  due  to  its  being  taken  up  in 
conjunction  with  potassium.  Water-culture  experiments 
show,  however,  that  in  many  cases  it  cannot  be  omitted 
altogether  without  injury  to  the  plant.  It  has  been  asso- 
ciated by  some  writers  with  the  translocation  of  carbo- 
hydrates, particularly  in  the  buckwheat,  a  view  which  is 
based  upon  the  observation  that  in  its  absence  the  chloro- 
plasts  become  abnormally  filled  with  granules  of  starch. 
Bromine  and  iodine  are  chiefly  found  in  marine  plants, 
but  their  function  is  unknown. 

Manganese  is  a  constituent  of  many  plants.  Till  quite 
recently  nothing  was  known  about  its  influence  on  meta- 
bolism, but  it  now  appears  probable  it  plays  a  part  in 
various  oxidative  processes  which  are  carried  out  by  a 


THE  CONSTITUENTS  OF  THE  ASH  OF  PLANTS    187 

somewhat  widely  spread  enzyme  known  as  Laccase,  whose 
normal  function  is,  however,  at  present  obscure. 

The  elements  of  the  last  group  are  numerous  ;  they 
vary  with  the  composition  of  the  soil  in  which  the  plants 
are  growing,  and  appear  to  subserve  no  useful  purpose. 
Many  of  them  in  even  moderately  dilute  solutions  are 
extremely  poisonous,  so  that  they  must  be  absorbed  in  a 
high  state  of  tenuity.  Their  presence  shows  that  the 
selective  power  of  plants  is  not  necessarily  connected  with 
the  development  of  normal  metabolic  functions,  but  is 
mainly  physical  and  only  to  a  slight  extent  physiological. 

From  what  has  already  been  advanced,  it  is  evident 
that  the  time  is  not  ripe  for  a  detailed  discussion  of  the 
parts  played  by  the  constituents  of  the  ash  of  plants.  Nor 
will  it  be  till  we  have  ascertained  much  more  fully  how 
the  various  metabolic  processes  are  carried  on.  Certain 
broad  statements  of  a  somewhat  general  character  are  all 
that  are  at  present  justified,  and  these  concern  only  some 
of  the  mineral  matters  which  are  absorbed.  The  meta- 
bolism not  only  depends  on  the  presence  of  certain  elements, 
but  is  largely  influenced  by  the  relative  quantities  of  each 
which  the  active  cells  contain. 

The  vegetative  activity  of,  at  any  rate,  herbage  plants 
is  associated  with  a  plentiful  supply  of  nitrogen.  In  the 
absence  of  sufficient  potassium  vegetative  luxuriance  may 
be  obtained,  but  the  degree  of  development  of  the  plant 
is  limited  by  such  deficiency.  In  the  event  of  sufficient 
supplies  of  potassium  being  afforded,  the  relative  abundance 
of  the  nitrogen  has  an  important  influence  on  the  formation 
of  carbohydrates,  which  are  then  produced  in  greater 
quantities.  Coincidently  the  plants  go  on  to  maturity  ; 
the  luxuriance  of  the  leafy  parts  becomes  curtailed,  and 
the  development  proceeds  normally,  leading  to  the  forma- 
tion of  the  flowers  and  subsequently  the  seeds.  Thus  the 
composition  of  the  supplies  in  the  soil  determines  largely 
the  character  of  the  development  of  the  plants  growing 
in  it.  It  has  also  considerable  influence  upon  the  variety 


188  VEGETABLE  PHYSIOLOGY 

of  the  species  of  these  plants,  owing  to  the  various  ways 
in  which  particular  constituents  may  influence  different 
individuals. 

In  the  absence  or  the  deficiency  of  particular  salts,  others 
may  be  absorbed  in  proportions  very  different  from  those 
which  would  have  been  found,  had  the  missing  element 
or  elements  been  present. 


189 


CHAPTEK  XIII 

OTHER  METHODS  OF  OBTAINING  FOOD 

In  our  introductory  considerations  of  the  true  nature  of 
the  food  of  plants,  and  of  the  manner  in  which  they  obtain 
it,  it  was  pointed  out  that  there  are  stages  in  the  life- 
history  of  all  plants  during  which  it  is  imperative  that  they 
shall  be  supplied  with  food  in  a  form  in  which  they  can 
assimilate  it  at  once,  constructive  mechanisms  either  being 
altogether  absent  from  them  or  not  having  been  developed 
at  the  particular  time  under  consideration.  There  is  thus 
in  every  plant  a  power  of  assimilating  organic  food  so 
supplied,  a  power  which  in  some  cases  is  permanently 
relied  upon,  sometimes  completely,  sometimes  only  partially, 
and  which  in  other  cases  is  laid  aside  as  soon  as  the 
chlorophyll  apparatus  becomes  developed.  The  need  for 
the  supply  of  the  organic  food  is  always  felt  by  every 
protoplast,  and  the  latter  cannot  be  nourished  except  by  it. 
We  may  contrast  in  this  respect  the  individual  proto- 
plast and  the  colony  of  which  it  is  a  member,  the  latter 
being  able  through  the  co-operation  of  its  individuals  to 
construct  the  organic  food  which  must  be  provided  for 
the  use  of  every  member,  even  of  those  to  which  the  work 
of  construction  is  allotted. 

The  constructive  power  may  be  partially  or  wholly  lost 
or  undeveloped  ;  in  such  cases  the  loss  must  be  compensated 
for  by  the  supply  from  outside  of  the  material  the  plant  is 
not  able  to  synthesise  for  itself. 

Examples  of  plants  possessing  different  powers  of  such 


190  VEGETABLE  PHYSIOLOGY 

absorption  are  supplied  by  every  class  of  the  vegetable 
kingdom.  They  are  most  conspicuous  among  the  Fungi, 
because  in  them  there  is  no  chlorophyll  apparatus,  and 
hence  constructive  processes  must  be  very  rudimentary. 
Distinct  differences  can  be  seen,  however,  in  this  group. 
Certain  lowly  forms  appear  to  be  able  to  utilise  very 
different  compounds  of  carbon  and  to  synthesis e  carbo- 
hydrates therefrom.  Many  can  grow  and  multiply  in 
solutions  of  simple  acids  such  as  formic  or  acetic.  A  much 
larger  number  need  for  their  nutriment  a  supply  of  carbo- 
hydrates in  the  form  of  sugar,  and  if  this  is  given  them, 
together  with  certain  relatively  simple  compounds  of 
ammonia,  especially  ammonium  tartrate,  they  can  con- 
struct therefrom  protein  and  fatty  bodies. 

Others  need  the  nitrogen  to  be  supplied  in  the  form  of 
amino-  or  amido-acids,  as  they  have  no  power  to  utilise 
the  simpler  ammonium  salts  ;  others  again  need  their  pro- 
teins as  well  as  their  carbohydrates  to  be  supplied  to  them 
as  such,  for  they  possess  scarcely  any  constructive  ability. 

A  similar  power  of  utilising  carbohydrates  and  allied 
bodies  is  exhibited  by  many  green  plants.  If  their  roots 
are  watered  with  a  solution  of  sugar,  they  can  take  it  up 
and  economise  by  its  aid  the  sugars  which  the  chlorophyll 
apparatus  is  constructing.  Various  bodies  also  from  which 
sugar  can  be  formed  are  absorbed  when  presented  to 
the  roots  and  serve  as  forerunners  of  sugar  in  the  plant. 
Among  these  may  be  mentioned  Glycerine.  The  process 
of  the  synthesis  of  proteins  also  may  be  shortened  by  sup- 
plying the  roots  with  material  such  as  asparagin,  leucin, 
or  urea.  Protein  as  such  can  only  be  utilised  by  a  few 
flowering  plants  which  possess  special  mechanisms  for  its 
preliminary  digestion. 

Among  what  we  must  regard  as  these  abnormal  methods 
of  food  supply  we  must  include  certain  processes  in  which  two 
organisms  are  associated,  for  the  well-being,  in  some  cases, 
of  both,  in  others  for  that  of  only  one.  The  two  organisms 
are  brought  into  very  intimate  relationships  with  each 


OTHEK  METHODS  OF  OBTAINING  FOOD      191 

other,  in  some  cases  a  very  complete  union  of  their  tissues 
being  effected,  so  that  transport  of  elaborated  food  materials 
can  readily  take  place  between  them.  In  those  cases  in 
which  this  close  association  is  of  benefit  to  both  the 
organisms  it  is  spoken  of  as  symbiosis ;  in  those  in  which 
one  flourishes  at  the  expense  of  the  other,  the  relationship 
is  called  parasitism.  While  there  are  many  cases  which 
can  be  definitely  referred  to  both  these  categories,  they 
seem  to  blend  one  into  the  other,  cases  being  known  in 
which  it  is  very  difficult  or  impossible  to  say  whether  the 
advantages  are  all  on  one  side  or  not. 

The  plants  which  differ  least  from  the  normal  habit, 
which  we  have  described,  are  those  which  are  known  as 
Saprophytes,  their  characteristic  feature  being  that  they 
derive  at  least  part  of  their  food  from  decaying  animal  or 
vegetable  matter,  absorbing  it  in  some  cases  as  actual  food- 
stuffs, and  in  others  as  organic  compounds  which  require 
relatively  little  expenditure  of  energy  to  build  them  up 
again  into  proteins  or  carbohydrates. 

Numerically  the  fungi  are  the  most  prominent  in  this 
group,  but  the  green  plants  also  afford  many  instances  of 
the  habit.  Among  the  mosses  Splachnum  can  grow  upon 
lumps  of  dung,  and  various  species  of  Hypnum  flourish  in 
water  which  contains  various  compounds  derived  from 
the  decomposition  of  once  living  matter.  Among  higher 
plants  still,  the  soil  of  woods  and  pastures  affords  many 
examples  of  individuals  which  depend  partly  upon  the  humus 
of  the  soil  and  partly  on  their  own  chlorophyll.  Among  the 
ferns  we  have  notably  the  moon-wort,  Botrychium  Lunaria, 
and  among  the  club-mosses  some  species  of  Lycopodium, 
while  numerous  flowering  plants  show  this  peculiarity. 

The  chlorophyll  apparatus  is  found  in  nearly  all  of  them, 
though  in  some  cases  it  is  so  reduced  as  to  be  almost  function- 
less.  Some  of  our  native  Orchids  are  remarkable  in  this 
respect,  that  they  are  almost,  if  not  altogether,  dependent 
upon  their  saprophytism.  Neottia,  the  so-called  bird's-nest 
orchis,  has  a  flowering  stem  above  ground,  on  which  are  only 


192  VEGETABLE  PHYSIOLOGY 

a  few  rudimentary  leaves.  At  the  base  of  the  stem  there 
is  found  a  cluster  of  fairly  stout  root-like  structures  which 
intertwine  with  each  other  to  form  a  mass  sometimes  as 
large  as  a  man's  fist.  These  are  developed  only  in  masses 
of  humus,  from  which  they  absorb  the  products  of  decay. 
These  plants  differ  thus  from  normal  phanerogams  by  their 
method  of  absorbing  food.  Their  subterranean  members 
are  not  provided  with  the  system  of  short-lived  root-hairs 
which  are  so  characteristic  of  the  ordinary  roots.  They  are 
not  in  need  of  such  close  contact  with  continually  fresh 
particles  of  soil  as  are  the  latter,  lying  as  they  do  embedded 
in  a  mass  of  humus.  In  some  cases  all  their  external  cells 
absorb  material  from  this ;  in  others  special  absorptive  cells 
are  present,  but  these  are  not  localised  like  the  ordinary 
root-hairs,  and  they  are  not  being  continually  renewed,  but 
remain  active  for  long  periods.  Frequently  they  are  only 
found  at  the  points  where  contact  with  the  humus  is  effected. 
Many  of  these  saprophytes  have  the  cells  of  their  cortex 
infested  with  the  hyphse  of  a  fungus. 

The  food  which  is  thus  absorbed  from  the  decaying  organic 
matter  is  not  necessarily  in  a  fit  condition  for  immediate 
assimilation  by  the  protoplasts.  It  may,  and  frequently 
does,  require  alteration  before  being  available  for  nutrition. 

The  plants  of  the  next  group  which  we  must  consider 
differ  from  the  saprophytes  in  an  important  particular. 
Like  them  they  are  provided  with  a  chlorophyll  apparatus, 
and  are  consequently  capable  of  carrying  on  carbohydrate 
construction.  Indeed  they  are  generally  more  active  in  this 
respect  than  the  members  of  the  last  group.  As  in  the  case 
of  the  greater  number  of  the  latter,  it  is  chiefly  their  nitro- 
genous material  that  they  obtain  nearly  or  quite  ready  for 
assimilation.  They  appear  to  need  this  nitrogenous  food 
in  the  form  of  proteins,  and  they  obtain  it  by  capturing 
and  killing  various  animal  organisms  whose  putrefying 
bodies  yield  them  what  they  want. 

The  Dtricularias,  which  are  members  of  this  group,  are 
plants  which  live  floating  in  water  (fig.  91) ;  they  have  a 


OTHEB  METHODS  OF  OBTAINING  FOOD    193 

much-branched  stem  which  bears  a  number  of  leaves,  the 
shapes  of  which  differ  in  the  case  of  different  species  ;  they 


possess  no  roots.  Growing  out  of  the  stems  are  numerous 
small  bladder -like  bodies,  each  with  a  small  opening  at  its 
apex.  This  orifice  is  guarded  by  a  number  of  stiff  tapering 

13 


194 


VEGETABLE  PHYSIOLOGY 


bristles,  and  is  closed  by  a  sort  of  trapdoor  which  opens 
inwards  and  shuts  again  with  a  kind  of  spring.  A  small 
animal  such  as  an  aquatic  insect  can  easily  open  it  by  press- 
ing against  it,  and  thus  can  enter  the  bladder.  The  trapdoor 
immediately  closes  by  virtue  of  its  own  elasticity,  and 
cannot  be  opened  by  pressure  from  within.  The  insect 


FIG.  92. — TRAPS  OF  Utricularia  neglecta.     (After  Keruer.) 

a,  a  bladder  magnified  ( X  4) ;   b,  section  of  a  bladder ;   c,  absorption-cells 
on  the  internal  surface  of  the  bladder  (  x  250). 


accordingly  finds  egress  impossible,  and  after  a  short  time, 
usually  ranging  from  one  to  three  days,  it  perishes  and  its 
body  decays,  yielding  to  the  plant  the  products  of  its  decom- 
position, which  are  absorbed  by  particular  cells  growing 
from  the  internal  wall  of  the  bladder  (fig.  92). 

Some  of  the  so-called  pitcher-plants  show  a  somewhat 
similar  mechanism  and  utilise  corresponding  organic  sub- 
stances. The  Sarracenias  afford  good  examples.  These 


OTHER  METHODS  OP  OBTAINING  FOOD     195 


are  marsh  plants  having  their  leaves  arranged  in  rosettes, 
which  spring  apparently  from  the  surface  of  the  soil,  and 
from  the  centre  of  which  arises  a  single  flower-stalk.  Each 
leaf  is  modified  to  form  a  curious  pitcher-like  body  (fig.  93), 
furnished  with  a  kind  of  lid. 
The  pitchers  are  generally  con- 
spicuously coloured,  while  the  lid, 
which  is  the  lamina  of  the  leaf, 
often  bears  hairs  which  secrete 
honey  to  attract  the  prey. 

The  inner  surface  of  the  pitchers 
is  lined  with  slippery  recurved  hairs 
which  make  it  impossible  for  an 
insect  to  climb  out  of  it  after  once 
entering.  Insects  are  attracted  by 
the  honey,  and,  venturing  upon 
these  hairs,  slip  down  to  the  bottom 
of  the  pitcher,  from  which  escape 
is  impossible.  The  pitcher  contains 
a  quantity  of  water,  due  perhaps  to 
the  entrance  of  rain,  or  possibly 
secreted  to  some  extent  by  the 
surface  of  the  pitcher.  The  insects 
become  drowned  in  this  liquid  and 
undergo  decomposition.  Frequently 
a  pitcher  will  contain  so  many  that 
the  products  of  their  putrefaction 
are  offensive.  They  are  absorbed 
by  the  cells  of  the  inner  surface. 

Certain  other  pitcher-plants 
show  a  still  further  advance  in  their  method  of  obtaining 
protein  supplies.  They  possess  similar  means  of  attracting 
insects  and  alluring  them  to  their  death,  but  they  do  not 
depend  on  the  slow  process  of  putrefaction  for  the  decom- 
position of  their  prey.  Instead  of  this,  they  secrete  and 
pour  out  a  definite  digestive  fluid  possessing  properties  like 
those  of  the  secretions  of  the  stomach  and  pancreas  of 

13* 


FIG.  93. — LEAF  OF  Sarracenia, 

MODIFIED         TO        FORM        A 

PITCHEK.      (After   Kerner.) 


196 


VEGETABLE  PHYSIOLOGY 


the  higher  animals,  by  the  instrumentality  of  which  the 
insoluble  proteins  of  their  prey  are  converted  into  pep- 
tones, and  possibly  partially  into  ammo-  and  amido-acids, 

prior  to  actual  absorption. 
Among  these  Nepenthes  may 
be  mentioned. 

The  pitchers  of  Nepenthes 
(tig.  94)  are  in  the  main 
similar  to  those  of  Sarra- 
cenia.  They  possess  means  of 
attracting  insects  to  them,  of 
seducing  them  into  the  in- 
terior of  the  pitcher,  and  of 
preventing  their  subsequent 
escape,  all  of  which  are  com- 
parable to  those  already  de- 
scribed. The  pitchers  con- 
tain a  watery  liquid,  which 
is  secreted  by  their  interior 
surfaces,  and  which  has  a 
faintly  acid  reaction.  When 
an  animal  is  captured  and 
falls  into  the  liquid,  it  sets 
up  a  further  secretion,  which 
is  more  strongly  acid,  and 
which  contains  a  peculiar 
body  known  as  an  enzyme 
or  ferment,  the  properties  of 
which  will  be  discussed  in  a 
subsequent  chapter.  This 

ferment  somewhat  closely  resembles  the  active  principles  of 
the  gastric  and  pancreatic  juices  of  the  human  body,  and  in 
the  acid  medium  is  capable  of  converting  the  proteins  of  the 
prey  into  peptone,  leucin,  and  tyrosin,  products  which  are 
all  soluble  and  diffusible.  This  secretion  is  prepared  by 
special  glands,  which  are  plentifully  distributed  over  the 
lower  portion  of  the  internal  face  of  the  pitcher. 


FIG.  94. — MODIFIED   LEAF   (PITCHER) 
OF  Nepenthes.     (After  Kerner.) 


OTHER  METHODS  OF  OBTAINING  FOOD     197 

There  are  other  plants  which  effect  the  capture  and 
digestion  of  insects  in  other  ways.  Drosophyllum,  which 
is  found  in  part  of  the  Mediterranean  region,  is  furnished 
with  a  number  of  long  filiform  leaves,  which  are  closely 
set  with  stalked  glands.  These  pour  out  a  peculiar  muci- 
laginous secretion  which  forms  a  drop  of  very  glistening 
appearance  round  their  swollen  heads.  There  are  other 
sessile  glands  among  them  which  exude  an  acid  digestive 
secretion  resembling  the  gastric  juice  of  the  stomach,  when 
they  come  into  contact  with  protein  animal  matter.  An 
insect,  attracted  to  the  leaves  by  their  glistening  appearance, 
is  at  once  entangled  in  the  viscid  mucilage  and  is  presently 
suffocated.  It  is  speedily  digested  by  the  secretion  of  the 
sessile  glands. 

Pinguicula,  the  butter-wort,  has  a  mechanism  of  a 
somewhat  similar  nature.  It  bears,  resting  on  the  ground, 
large  fleshy  green  leaves,  the  edges  of  which  are  slightly 
curled  over  towards  the  upper  surface,  forming  a  kind  of 
open  trough.  All  over  the  upper  surface  are  distributed 
glands  which  pour  out  a  viscid  mucilage.  On  contact  with 
any  small  mass  of  protein,  or  with  an  insect  or  other  small 
animal,  these  glands  also  pour  out  an  increased  amount  of 
mucilage,  mixed  now  with  a  digestive  fluid  similar  to  that 
of  Drosopliyllum.  If  an  insect  alights  upon  the  margin  of 
the  leaf,  not  only  is  the  secretion  poured  out,  but  the  edge 
slowly  curls  over  more  strongly,  either  covering  the  intruder, 
or  pressing  it  towards  the  centre  of  the  trough.  Here  it  ia 
suffocated  and  digested  as  in  other  cases.  Pinguicula  is 
peculiar  in  that  its  secretion  has  the  power  of  curdling  milk 
in  the  same  way  as  the  gastric  juice  of  animals. 

In  some  cases  a  yet  more  elaborate  mechanism  is  found 
to  effect  the  same  purpose.  We  find  associated  with  the 
power  of  digesting  and  absorbing  animal  food,  a  mechanism 
for  the  capture  of  the  prey  which  involves  a  movement  of 
either  the  leaf -blade  itself  or  of  the  glands  which  it  pro- 
duces. The  former  is  exhibited  by  Dioncea,  the  Venus's 
fly-trap  ;  the  latter  by  the  different  species  of  Drosera  (the 
Sundews). 


198 


VEGETABLE  PHYSIOLOGY 


Drosera  is  a  small  plant  which  is  found  growing  upon  a 
substratum  of  bog-moss  (Sphagnum).  Its  dimensions  are 
small,  the  plant  not  being  more  than  a  few  inches  in  height. 
It  bears  a  rosette  of  leaves,  from  the  middle  of  which  rises  a 
single  scape  of  flowers,  The  leaves  are  covered  with  stalked 


FIG.  95. — LEAF  OF  Drosera,  SHOWING  THE  GLANDULAR  TENTACLES. 


glands  (fig.  95),  which  stand  out  from  the  surface.  Each 
gland  has  a  somewhat  substantial  stalk,  containing  a  rudi- 
mentary vascular  bundle.  At  the  top  of  the  stalk  is  a 
rounded  head  which  is  always  covered  by  a  viscid  secretion 
that  it  pours  out.  Erom  the  shining  appearance  of  the 
glands  with  their  drops  of  mucilage,  the  name  of  the  plant, 
Sundew,  is  derived.  When  an  insect  alights  upon  the  leaf 
it  is  entangled  in  the  secretion,  and,  struggling  to  be  free,  is 


OTHEK  METHODS  OF  OBTAINING  FOOD     199 

brought  into  contact  with  more  and  more  of  the  drops,  be- 
coming hopelessly  captured.  The  stimulus  of  contact  pro- 
vokes a  movement  of  the  stalked  glands,  all  of  which  slowly 
bend  over  and  bring  their  viscid  heads  to  bear  upon  the 
struggling  insect.  The  same  disturbance  causes  an  outflow 
of  acid  enzyme-containing  secretion,  which  surrounds  the 
prey,  and  digestion  and  absorption  follow  as  before.  After 


2b 


FIG.  96. — LEAF  OF  Dioncea  muscipula. 

1,  open ;   2,  closed  :  a,  lateral  view,  b,  surface  view ;   3,  one  of  the  sensitive 
spines  (  x    60) ;  4,  glands  on  the  surface  of  the  leaf  (_x  100). 

a  time  the  glands  unfold  again  and  resume  their  normal 
attitude,  and  the  leaf  is  ready  to  receive  another  visitor. 

Dionaea  affords  an  instance  in  which  the  movement  of 
capture  is  effected  with  greater  rapidity.  Like  most  of  the 
insectivorous  plants  it  possesses  a  rosette  of  leaves  which 
rest  upon  the  ground,  and  from  the  centre  of  the  rosette 
it  gives  off  a  single  inflorescence.  The  leaves  are  very 
different  from  those  of  Drosera.  They  have  a  flat  ex- 
panded petiole,  at  the  end  of  which  the  lamina  is  attached 


200  VEGETABLE  PHYSIOLOGY 

by  a  sort  of  joint.  The  lamina  is  roundish  and  is  divided 
into  two  almost  exactly  similar  halves,  which  are  separated 
by  the  midrib  (fig.  96).  The  edge  of  each  half  is  furnished 
with  a  number  of  rigid  teeth,  and  when  the  two  halves  are 
folded  together  on  a  hinge  which  the  midrib  forms,  the 
teeth  interlock  with  each  other  and  a  closed  cavity  is  pre- 
pared. On  the  upper  surface  of  each  half  of  the  leaf,  about 
in  the  centre,  are  three  short  spines  which  project  out- 
wards and  upwards.  When  either  of  these  is  touched  twice 
in  rapid  succession,  the  two  lobes  of  the  lamina  become 
slightly  concave  and  fold  over  quickly,  the  teeth  interlock, 
and  the  cavity  is  closed.  If  the  contact  has  been  made  by 
an  insect,  it  is  captured  and  imprisoned  between  the  lobes. 
The  closing  is  fairly  rapid,  taking  only  about  a  second.  All 
over  the  upper  surface  of  the  lamina  secreting  glands  are 
found,  whose  secretion  is  similar  to  that  of  Drosera.  If 
the  leaf  encloses  nitrogenous  digestible  matter,  such  as  the 
body  of  an  insect,  the  prison  remains  closed  for  some  con- 
siderable time,  and  the  glands  surround  the  prey  with 
the  digestive  fluid,  the  products  of  its  decomposition  being 
absorbed  by  the  gland-cells. 

These  mechanisms  for  the  digestion  and  absorption  of 
protein  substances  are,  seen  to  be  extremely  complex.  Evi- 
dence of  such  digestion  and  absorption  is  shown  also  by  far 
humbler  plants  without  any  differentiated  structure.  Many 
Fungi  and  Bacteria,  when  cultivated  in  solutions  containing 
native  proteins,  such  as  albumin  or  globulin,  are  able  to 
effect  their  digestion  by  the  secretion  of  a  similar  enzyme  to 
those  of  the  plants  already  described.  They  subsequently 
absorb  the  peptone  or  the  amino- acids  which  result  from 
such  action.  Nor  is  protein  material  alone  affected  in  this 
way  by  these  humbler  plants.  They  derive  their  carbo- 
hydrate supplies  from  their  environment  in  the  same  way 
as  their  protein  ones.  Many  of  the  filamentous  fungi 
possess  the  property  of  forming  digestive  enzymes,  which 
attack  in  some  cases  starch,  in  others  inulin,  in  others 
various  sugars  which  are  not  immediately  available  for 


OTHEE  METHODS  OF  OBTAINING  FOOD    201 

nutrition,  in  yet  others  certain  more  complex  substances, 
all  of  which  undergo  this  external  process  of  digestion,  the 
resulting  bodies  being  subsequently  absorbed. 

In  the  earlier  pages  of  this  chapter  we  drew  attention 
to  the  fact  that  it  was  not  at  all  uncommon  to  find  two 
plants  closely  associated  together,  with  different  degrees  of 
completeness,  with  a  view  to  their  co-operation  in  carrying 
out  some  of  these  abnormal  processes  of  nutrition.  We 
may  now  study  these  relationships  a  little  more  fully. 

The  simplest  cases  of  the  dependence  of  one  plant  upon 
another  are  afforded  by  the  so-called  epiphytes,  repre- 
sentatives of  which  are  supplied  by  many  members  of  the 
Orcliidacece  and  the  Bromeliacece  which  inhabit  tropical 
forests.  The  dependence  in  these  cases  is  merely  one  of 
situation.  The  epiphyte  grows  upon  the  external  surface 
of  some  supporting  tree,  to  which  it  clings  by  various 
arrangements,  without  penetrating  into  its  tissues.  Fre- 
quently the  long  roots  of  the  epiphyte  are  attached  closely 
to  the  crannies  of  the  bark  of  the  tree,  and  the  dust  and 
debris  which  accumulate  there  are  utilised  for  the  purpose 
of  supplying  it  with  nutriment.  In  other  cases  the  support- 
ing plant  does  not  give  it  even  so  much  assistance. 

An  almost  equally  simple  relationship  is  seen  in  the  cases 
of  Anthoceros  and  Azolla.  Cavities  in  the  tissues  of  these 
plants  are  inhabited  by  numerous  cells  of  an  Alga  (Nostoc  or 
Anabcena).  Beyond  affording  them  shelter  and  a  certain 
degree  of  protection,  the  higher  plant  does  nothing  for  its 
guests.  The  relationship  is  sometimes  called  commensalism. 

A  more  complete  association,  attended  by  distinct  advan- 
tage to  one  or  both  of  the  plants  taking  part  in  it,  is  known 
under  the  name  of  symbiosis.  By  some  writers  this  term  is 
confined  to  such  an  association  as  is  of  benefit  to  both 
organisms,  and  does  not  profit  one  at  the  expense  of  the 
other.  Where  the  latter  is  the  case  the  relationship  is  said 
to  be  one  of  more  or  less  complete  parasitism.  Others  speak 
of  reciprocal  and  antagonistic  symbiosis,  to  indicate  these 
two  different  kinds  of  association. 


202 


VEGETABLE  PHYSIOLOGY 


One  of  the  best  known  cases  of  symbiosis  in  the  strict 
sense  is  that  of  the  Lichens.  These  are  lowly  organisms 
which  are  epiphytic  upon  tree-trunks,  old  walls,  rocks,  and 
other  supporting  structures.  They  are  composed  always  of 
two  distinct  plants,  an  Alga  and  a  Fungus,  which  are  closely 
united  together  to  form  a  kind  of  thallus  (fig.  97).  The 
relative  modes  of  arrangement  differ  in  different  species, 
and  many  algae  and  many  fungi  are  found  to  be  capable  of 
entering  into  such  an  association.  The  advantages  which 
result  to  the  two  constituents  of  the  lichen  are  consider- 
able. The  alga,  which  possesses  chlorophyll,  is  able  to  con- 
struct carbohydrate  materials  by  its  instrumentality,  and 
after  their  formation  these  are  shared  by  the  fungus, 

which  has  no  such  construc- 
tive powers.  The  fungus  is 
able  to  condense  aqueous 
vapour,  which  is  very  neces- 
sary in  the  dry  situations 
lichens  occupy.  It  can  thus 
dissolve  much  of  the  dust 
and  other  debris  of  its  rest- 
ing place,  and  so  carry  raw 
material  to  the  constructive 
algal  cells.  It  also  attaches 
the  thallus  to  the  substratum. 
Both  partners  can  no  doubt 
take  part  in  the  construc- 
tion of  proteins.  The  rela- 
tionship affords  a  further 
advantage-,  for  the  compound 
organism  is  much  better  able 
than  either  of  its  separate 
resist  adverse  conditions  of  temperature, 


FIG.  97.  —  SECTION  OF  A  LICHEN 
SHOWING  ALGAL  CELLS  (g)  IN  THE 
MIDST  OF  A  NETWORK  OF  FUNGAL 
HYPH;E  (m).  (After  Sachs.) 


constituents  to 
drought,  &c. 

A  similar  symbiosis  is  met  with  in  the  so-called  kephir 
organism  and  others  of  the  same  kind.  In  these  cases  the 
two  constituents  are  a  yeast  and  a  bacterium,  the  former  of 


OTHEE  METHODS  OF  OBTAINING  FOOD    203 

which  is  closely  surrounded  by  chains  of  the  latter,  making 
a  fleshy  mass  of  irregular  shape,  and  sometimes  of  compara- 
tively conspicuous  dimensions.  The  parts  played  by  Iho 
two  organisms  are  not  very  well  understood,  but  there 
seems  to  be  no  doubt  that  the  association  is  mutually 
beneficial. 

In  a  former  chapter  mention  was  made  of  a  property 
which  is  possessed  under  certain  conditions  by  various 
plants,  particularly  by  some  members  of  the  Leguminosce — 
that  of  being  able  to  utilise  the  free  nitrogen  of  the  air 
in  the  construction  of  protein  food-substances.  The  power 
was  shown  to  be  connected  with  the  formation  of  certain 
tubercular  structures  upon  the  roots  of  the  leguminous 
plant.  These  tubercles  are  swellings  of  the  cortex  of  the 
root,  the  cells  of  which  are  inhabited  by  a  particular  fungus, 
which  breaks  up  in  their  interior  into  curious  bacterioid 
bodies.  The  exact  nature  of  the  fungus  has  not  been 
accurately  determined.  The  soil  contains  many  of  these 
bacterium-like  bodies,  which  make  their  way  into  the  interior 
of  the  leguminous  plants  by  penetrating  their  root-hairs,  and 
growing  down  them  into  the  cortex  of  the  root.  In  the  cells 
of  the  latter  the  penetrating  filaments  bud  off  the  bacterioid 
bodies  in  great  numbers.  The  stimulus  resulting  from  the 
invasion  causes  a  considerable  hypertrophy  of  the  cortex  of 
the  roots  at  the  points  attacked,  and  tubercles  are  frequently 
the  result.  The  fungus  appears  to  have  the  power  of  fixing 
atmospheric  nitrogen,  bringing  it  into  some  combination, 
the  exact  nature  of  which  is  unknown,  but  which  serves  as 
the  starting  point  of  protein  synthesis,  either  by  the  green 
plant  or  by  the  intruder.  The  relationship  is  clearly  of 
great  advantage  to  both  organisms,  the  fungus  obtaining  its 
carbohydrate  supplies  from  the  green  plant,  much  as  is  the 
case  in  the  lichens  already  described. 

A  somewhat  similar  symbiosis  is  met  with  in  the  roots  of 
Cycas.  A  regular  system  of  spaces  in  the  cortex,  extending 
round  it  in  an  almost  regular  cylinder,  is  occupied  by  an 
assemblage  of  Algae  (Andbcena)  and  Bacteria  (Pseudomonas 


204 


VEGETABLE  PHYSIOLOGY 


and  Azobader)  which  apparently  have  the  power  of  fixing 
atmospheric  nitrogen. 

Many  of  our  forest  trees,  among  which  the  members  of 
the  Cupuliferce  are  conspicuous,  exhibit  another  symbiosis 
which  is  of  the  greatest  interest  and  importance.  The 
roots  of  these  plants  grow  down  into  soil  which  is  infested 
with  the  mycelia  of  different  fungi,  with  which  they  become 
entangled.  The  hyphae  of  the  fungi  continue  to  grow 

together  with  the  root, 

S  A  and    form  an  invest- 

ment over  it,  which 
is  in  some  cases  met 
with  in  the  form  of  an 
open  network,  and  in 
others  in  that  of  a 
dense  feltwork  (fig. 
98).  The  fungi  in 
some  peases  perforate 
the  external  cells  of 
the  roots  and  form 
a  network  in  the  in- 
terior. From  the  out- 
side of  the  investing 
mantle  hyphae  grow 
out  into  the  soil  in 

a  similar  way  to  the  root-hairs  of  ordinary  plants.  These 
take  the  place  of  the  root-hairs,  which  cease  to  be  developed, 
and  serve  the  purposes  of  the  roots  as  absorbing  organs 
for  the  water  and  the  salts  of  the  soil.  The  fungus  is  bene- 
fited by  drawing  its  own  nutriment  from  the  cells  of  the  root 
into  which  it  has  penetrated.  The  fungoid  web  or  mantle 
is  known  as  a  mycorhiza  ;  it  is  present  not  only  on  the  roots 
of  the  Cupuliferae,  but  on  those  of  Poplars,  and  many  Heaths 
and  Ehododendrons. 

A  curious  case  of  this  kind  of  relationship  is  shown  by 
Monotropa,  a  member  of  the  Heath  family  which  possesses 
no  chlorophyll.  Monotropa  possesses  a  rhizome,  from  which 


Fro.   98. — A,    EPIPHYTIC   MYCORHIZA   OF    Facjus 
sylvatica  (  x  2) ;    B,  TIP  OF  HOOT  PARTIALLY 

DENUDED  OF  THE    INVESTING  MANTLE    (  X     30). 

(After  Pfeffer.) 


OTHEE  METHODS  OF  OBTAINING  FOOD     205 

rise  subaerial  stems  from  ten  to  twenty  centimetres  high, 
bearing  succulent  membranous  leaves.  From  the  rhizome 
are  given  off  crowded  masses  of  roots  which  are  covered 
with  a  mycorhizal  mycelium,  and  are  embedded  in  humus. 
There  being  no  chlorophyll  apparatus,  Monotropa  is  de- 
pendent entirely  on  the  mycorhiza  for  its  nourishment. 
The  latter  is  entirely  saprophytic.  We  have  here  a  curious 
case  of  the  complete  dependence  of  a  higher  plant  upon 
a  more  lowly  one. 

A  complete  symbiosis  between  two  green  plants  is  occa- 
sionally met  with.  A  good  instance  is  afforded  by  the 
Mistletoe  and  the  plants  upon  which  it  grows,  usually 
either  the  Poplar,  the  Silver  Fir,  or  the  Apple-tree.  The 
seed  of  the  Mistletoe  is  left  by  a  bird  upon  a  branch  of 
one  of  these  trees,  and  under  appropriate  conditions  it 
germinates.  The  root  of  the  seedling  penetrates  into  the 
bark  of  the  tree  and  grows  inwards  till  it  reaches  the  wood. 
It  makes  its  way  no  further,  but  maintains  its  position 
there,  and  as  the  branch  gradually  thickens  by  the  activity 
of  its  cambium,  the  intruding  root  is  by  degrees  impacted 
in  the  secondary  wood,  its  own  growth  preventing  its  being 
cut  off  and  buried  by  the  latter.  The  root  branches  in  the 
substance  of  the  tree,  and  the  secondary  roots  make  their 
way  along  in  the  bast,  growing  parallel  with  the  exterior. 
These  branches  also  put  out  small  vertical  outgrowths, 
which  make  their  way  to  the  wood  just  as  the  primary 
root  did.  A  very  complete  fusion  of  the  tissues  of  the  two 
plants  is  thus  ultimately  arrived  at.  The  advantage  of  the 
alliance  is  on  the  side  of  the  Mistletoe,  which  derives  a  great 
part  of  its  nourishment  from  the  host.  It  'possesses  ever- 
green leaves,  however,  which  serve  for  the  construction 
of  carbohydrates,  and  as  it  manufactures  these  during  the 
winter,  when  the  host  plant  has  no  leaves,  the  latter  is  able 
to  benefit  in  its  turn  during  that  season. 

Passing  on  to  notice  the  association  of  two  organisms 
which  is  known  by  the  name  of  antagonistic  symbiosis  or 
parasitism,  we  find  various  degrees  of  completeness  in  the 


206  VEGETABLE  PHYSIOLOGY 

dependence  of  one  form,  the  parasite,  upon  the  other,  the 
host.  As  in  the  case  of  the  insectivorous  plants,  there  are 
members  of  this  class  which  are  provided  with  a  chlorophyll 
apparatus,  and  which  are  therefore  indebted  to  their  hosts 
for  protein  substances  only,  or  perhaps  also  for  certain  of  their 
ash  constituents.  As  these  almost  without  exception  fasten 
themselves  upon  the  roots  of  the  host  plant,  they  are 
frequently  spoken  of  as  root-parasites.  From  their  general 


FIG.  99. — Thesium  alpinum,  SHOWING  THE  SUCKERS  ON  THE  ROOTS. 
(After  Kerner.) 

structure  and  their  relationship  to  the  host  plant,  they 
evidently  have  much  in  common  with  the  Mistletoe,  and 
it  is  not  very  easy  to  distinguish  between  their  semi-para- 
sitism and  the  symbiosis  of  the  latter  with  the  trees  on 
which  it  lives.  They  are,  however,  usually  herbaceous 
forms,  and  can  therefore  be  of  no  use  to  the  host  plant  in 
the  winter.  Moreover,  most  of  them  ultimately  destroy  the 
root  on  which  they  have  fastened. 

These  root-parasites  are  mainly  members  of  the  Scrophu- 
lariacece  or  the  Santalacece.  As  a  rule,  they  are  herbaceous 
annuals,  though  there  are  some  perennial  species.  They 


OTHEK  METHODS  OF  OBTAINING  FOOD     207 

grow  from  seed  with  fair  rapidity,  the  root  of  the  seedling 

attaining  a  length  of  an  inch  in  two  or  three  days.     Shortly 

after  penetrating  the  soil,  the  main  root  puts  out  secondary 

branches,  which  make  their  way  parallel  to  the  surface. 

As  they  grow  chiefly  in  woods  or  among  herbage,  they 

speedily  encounter  the  roots  of  other  plants,  and  on  contact 

being  made  between  one  of  these  root-branches  and  a  root 

of  a  suitable  host,  a  curious  sucker-like  body  is  developed 

at  the  point  of  con- 

tact (fig.  99).     This  is 

a  kind   of   parenchy- 

matous  cushion,  which 

partly  surrounds   the 

host,    and    from    the 

inner  side  of  its  con- 

cavity certain  absorp- 

tion -  cells    grow    out 

and     penetrate     into 

the    former,    pushing 

their  way  until  they 

reach    the   centre    of 

the  invaded  root  (fig. 

100).  These  absorbing 

Organs    are  Often  eiTO-       FlG>  100.—  TAwitm  alpinum.     PIECE  OF  A  ROOT 

WITH    SUCKER    IN    SECTION.      x    35.      (After 

neously  spoken  of  as 


roots.      They   cannot 

properly  be  so  called,  as  they  are  developed  from  the  cortex 
of  the  rootlet,  and  not,  as  root-branches  are,  from  the  tissue 
of  the  pericycle.  They  are  best  spoken  of  as  haustoria, 
a  term  which  is  purely  physiological,  and  carries  with 
it  no  anatomical  significance. 

While  the  root  is  setting  up  this  relationship  with  a  host 
plant,  the  shoot  of  the  seedling  is  growing  normally.  Its 
leaves  and  other  subaerial  parts  are  well  developed  and 
discharge  their  appropriate  functions.  The  plants  would 
not  be  recognised  at  all  as  in  any  way  parasitic  without 
an  examination  of  the  subterranean  parts.  They  absorb 


208  VEGETABLE  PHYSIOLOGY 

certain  nutritive  materials  from  the  roots  on  which  they 
fix  themselves,  and  generally  destroy  them.  The  damage 
is,  however,  local,  and  does  not  involve  the  death  of  the 
host  plant.  Indeed,  many  of  these  root-parasites  do  so 
little  harm  to  the  latter  that  an  affected  host  is  often  not 
noticeably  different  in  appearance  from  a  neighbouring 
plant  of  the  same  species  which  is  not  attacked. 

The  perennial  forms  produce  fewer  suckers  or  haustoria 
which  only  function  for  one  year.  The  rootlets  usually 
bear  only  one  sucker  each,  and  when  it  has  ceased  to  act 
as  an  absorbing  organ  it  dies.  The  rootlet  grows  on,  and 
in  the  next  year  develops  a  new  sucker,  and  makes  a  fresh 
attachment. 

Some  of  these  root-parasites  are  also  saprophytic  in  then- 
habit,  bearing,  besides  the  suckers,  absorbing  hairs  on 
their  underground  stems,  which  come  into  relationship 
with  the  humus  of  the  soil. 

There  are  many  other  plants  which  are  parasitic  upon 
roots,  but  they  must  be  distinguished  from  those  we  have 
just  discussed,  on  account  of  the  greater  degree  of  their 
parasitism.  They  include  such  forms  as  Lathrcea  and  Oro- 
banche,  which  are  members  of  the  British  Flora.  Lathrsea 
obtains  food  by  becoming  parasitic  on  the  roots  of  trees, 
to  which  its  roots  attach  themselves  by  suckers,  much  in 
the  same  way  as  the  semi-parasites  already  described.  The 
host  plant  in  this  case  is  drawn  upon  for  carbohydrates 
as  well  as  proteins,  as  Lathraea  possesses  no  chlorophyll. 

Orobanche  resembles  Lathrsea  in  exhibiting  the  same 
degree  of  parasitism.  It  shows  certain  differences  of  struc- 
ture, and  it  does  not  attach  itself  exactly  in  the  same  way. 
It  derives  its  nutriment  entirely  from  its  host,  which  is  fre- 
quently a  herbaceous  plant.  The  different  species  of  the 
genus  infest  different  plants,  each  having  only  one  suitable 
host. 

Some  curious  parasites  which  are  met  with  in  the  tropics 
show  a  very  peculiar  method  of  attaching  themselves  to 
their  host  plant.  They  constitute  the  natural  order 


OTHEK  METHODS  OF  OBTAINING  FOOD     209 


Rafflesiacece.  The  em- 
bryo, after  emerging 
from  the  seed,  pene- 
trates the  cortex  of  its 
host,  usually  a  root, 
though  not  always,  and 
gradually  forms  a  hol- 
low cylinder  surround- 
ing its  woody  centre. 
This  sheathing  struc- 
ture is  composed  of 
rows  of  cells,  and  in 
appearance  resembles 
the  mycelium  of  a  fun- 
gus. Buds  arise  upon 
this  investment,  which 
eventually  burst  the 
cortex  above  them,  and 
protrude  through  the 
host  plant.  These,  in 
Rajflesia  itself,  develop 
a  single  flower  which, 
in  some  cases,  is  of 
enormous  size.  The 
plant  produces  no  out- 
growths of  any  kind  ex- 
cept the  buds  described. 
Other  genera  show  some 
modification  of  this 
structure,  but  exhibit 
exactly  similar  physio- 
logical peculiarities. 

Certain  other  para- 
sites which  resemble 
these  in  many  respects 
differ  in  attacking  only 


FIG.  101. — PLANT  OF   Mdampyrum   (COW-WHEAT) 

INFESTED    WITH   CuSCUta. 

14 


210 


VEGETABLE  PHYSIOLOGY 


their  hosts.     The   most    easily    observed   of  these  is   the 
Dodder  (Cuscuta),  which  often  attacks  the  cow- wheat  or  the 


Fro.  102.— SECTION  OF  STEM  OF  DICOTYLEDONOUS  PLANT  ATTACKED  BY 
HATJSTOEIA  OF  Cuscuta. 

clover  (fig.  101).     The  seed  when  germinating  puts  out  an 
embryo  which    has   no    cotyledons.       Germination    takes 


OTHEK  METHODS  OF  OBTAINING  FOOD       211 


place  on  the  ground,  and  the  embryo  grows  to  a  length  of 
about  an  inch.  Its  apex  attaches  itself  to  the  ground,  and 
the  free  portion  moves  round,  describing  a  sort  of  spiral  in 
the  air.  If  it  comes  in  contact  with  a  suitable  host,  ilr 
twines  round  it  after  the  fashion  of  a  tendril,  and  numerous 
suckers  are  developed  in  rows  at  the  points  of  contact. 
Haustoria  spring  from  these  suckers  and  penetrate  the  host, 
extending  inwards  till  they  reach  the  wood  (fig.  102).  The 
part  below  the  attachment  dies  shortly  after  this  relationship 
has  been  established,  and  the  parasite 
is  left  attached  to  the  host.  In  its 
further  growth  it  continues  to  twine 
around  the  latter,  putting  out  numer- 
ous branches,  which  also  form  similar 
coils,  so  that  the  host  is  completely 
immeshed  in  the  twining  stems  of  the 
parasite.  -The  latter  bears  no  leaves 
and  possesses  no  chlorophyll  in  any 
part,  so  that  it  derives  all  its  food  in 
fully  elaborated  form  from  the  tissues 
of  the  host.  Cuscuta  produces  num- 
bers of  flowers  on  its  branches,  and 
from  them  are  developed  fruits  and 
seeds.  The  parasitism  is  complete, 
and  the  relation  frequently  leads  to  the 
death  of  the  host  which  has'  been 
attacked. 

Parasitic  plants  are  very  frequently  met  with  among  the 
fungi  and  the  Bacteria.  The  former  penetrate  the  living 
cells  01  the  plant  they  infest,  or  in  a  few  cases  ramify  between 
them,  sending  haustoria  into  the  interior  of  the  cells  between 
which  the  mycelium  grows  (fig.  103).  They  make  use  of  the 
contents  of  the  cells,  destroying  and  absorbing  the  living  sub- 
stance as  well  as  any  formed  materials  which  may  be  present. 
In  many  cases  also  they  destroy  the  cell-walls,  and  utilise 
the  carbohydrate  materials  of  which  the  latter  consist. 
Their  ravages  only  cease  with  the  death  of  the  organism. 

U* 


FIG.  103. — CELLS  OF  POTATO 
PLANT  INFESTED  WITH 
Phytophthora. 

b,  hypha  running  between 
the  cells  and  sending 
haustoria  (a)  into  their 
interior. 


212  VEGETABLE  PHYSIOLOGY 

The  power  of  living  plants  to  assimilate  the  food  manu- 
factured by  others  is  taken  advantage  of  in  the  processes 
of  grafting  and  budding.  In  these  operations  a  slip  of 
a  particular  plant  is  inserted  into  a  wound  made  in  the  stem 
of  another  nearly  related  one,  and  the  two  are  closely  bound 
together.  The  graft  or  scion  comes  into  such  close  connec- 
tion with  the  stem  or  stock  that  the  food  which  is  contained 
in  the  cells  of  the  latter  passes  into  the  tissues  of  the  graft, 
which  thus  receive  their  nourishment.  After  a  longer  or 
shorter  time  the  two  become  so  completely  united  that  they 
live  subsequently  as  a  single  organism,  and  the  processes  of 
carbohydrate  and  protein  construction  proceed  as  in  a 
normal  plant. 


213 


CHAPTEK  XIV 

TRANSLOCATION    OF   NUTRITIVE    MATERIALS 

We  have  so  far  traced  the  ways  in  which  plants  receive 
their  food,  and  have  examined  the  processes  by  which  it  is 
appropriated.  In  some  cases,  indeed  in  the  vast  majority 
of  instances,  it  is  constructed  in  the  interior  of  the  plant 
by  certain  of  the  protoplasts  from  simple  inorganic  materials 
which  are  absorbed  from  the  environment.  In  green  plants 
this  construction  extends  to  all  the  substances  which  can  be 
termed  food.  In  plants  without  a  chlorophyll  apparatus 
the  construction  is  partial  only,  never  going  so  far  as  the 
formation  of  carbohydrates,  though,  when  these  are  supplied 
together  with  inorganic  compounds  of  nitrogen,  proteins 
and  fats  can  be  manufactured.  In  other  cases  the  con- 
structive processes  are  supplemented  by  the  absorption  of 
food  in  a  suitable  condition  for  nutritive  purposes,  while 
in  others,  again,  the  last  method  is  the  only  one  observable, 
all  constructive  power  being  absent. 

There  are  other  considerations,  which  must  be  briefly 
stated,  which  have  a  bearing  upon  this  subject.  The  con- 
ditions of  life  of  an  ordinary  green  plant  involve  a  great 
extension  of  the  original  constructive  process.  It  has  no 
definite  and  regular  times  at  which  it  can  take  in  a  certain 
quantity  of  food,  which  are  regulated  partly  by  the  needs 
of  the  organism  and  partly  by  the  mysterious  factor  which 
we  call  appetite.  Its  absorptive  processes  are  much  more 
under  the  influence  of  natural  phenomena,  such  as  the 
degree  of  illumination,  the  amount  of  warmth,  moisture, 
&c.,  which  it  is  receiving.  Periods  of  intermission  of 


214  VEGETABLE  PHYSIOLOGY 

irregular  duration  are  caused  by  differences  in  these  respects, 
even  during  an  ordinary  day,  and  still  more  by  the  alterna- 
tion of  day  and  night ;  in  the  case  of  perennial  plants  yet 
greater  disturbances  are  caused  by  the  succession  of  the 
seasons  of  the  year,  and  the  alterations  these  produce  in 
the  amount  of  foliage  which  the  plant  preserves  ;  weather 
and  its  vicissitudes  form  a  series  of  disturbing  influences. 
We  have  thus  the  certainty  of  failure  to  survive  in  the 
struggle  for  existence  unless  the  initial  absorptive  and 
constructive  processes  are  supplemented  by  others,  which 
in  some  way  shall  make  the  organism  indifferent  to  these 
changes  and  intermissions  of  supply,  and  capable  of  carry- 
ing out  true  nutritive  work,  when  the  initial  stages  of 
such  work  are  checked  or  suspended.  In  other  words, 
suitable  conditions  for  the  construction  of  food  being 
intermittent,  the  plant  must  accumulate  a  reserve  store  on 
which  it  can  subsist  during  the  periods,  short  or  prolonged, 
when  no  such  manufacture  is  possible. 

We  may  view  the  matter  from  a  slightly  different  stand- 
point, and  yet  come  to  the  same  conclusion.  The  processes 
of  absorption  in  a  plant  depend,  as  we  have  seen,  almost 
entirely  upon  physical  conditions.  Given  a  certain  amount 
of  carbon  dioxide  in  the  air,  and  a  certain  amount  of  water 
in  the  plant  to  which  that  air  has  access,  the  carbon  dioxide 
will  be  dissolved  according  to  the  power  of  the  water  to 
dissolve  it,  or — putting  it  more  technically — according  to  its 
coefficient  of  solubility.  In  the  presence  of  the  chlorophyll 
apparatus,  with  the  access  of  sunlight,  the  other  subsequent 
changes,  which  we  have  discussed,  lead  to  the  continuation 
of  the  absorption  of  the  gas.  This  is  the  case  again  with  the 
root  and  its  relations  to  the  soil.  The  process  of  absorption 
of  water  with  its  dissolved  substances  will  proceed  as  long 
as  certain  physical  conditions  obtain.  Thus  the  plant  is, 
on  the  whole,  rather  passive  than  active  in  the  initial  stages 
of  its  own  feeding,  exercising  no  inhibitory  power,  such 
as  that  which  in  an  animal  is  attendant  upon  a  failure  or 
cessation  of  appetite. 


TKANSLOCATION  OP  NUTEITIVE  MATEKIALS     215 

These  considerations  lead  us  to  the  conclusion  that  when 
the  absorption  of  food  or  food  materials  by  a  plant  is  pro- 
ceeding, the  probabilities  are  decidedly  in  favour  of  such 
an  absorption  being  much  greater  than  the  immediate 
need  for  direct  consumption.  The  constructive  process, 
followed  by  the  accumulation  of  its  products,  is  certainly 
the  leading  one  in  the  history  of  the  different  members 
of  the  vegetable  kingdom.  Most  of  it  is  ultimately  devoted 
to  the  increase  of  the  framework  which  attends  upon  the 
multiplication  of  the  protoplasts,  which  we  commonly  speak 
of  as  growth,  and  proceeds  for  such  long  periods  that  there 
is  accumulated  in  such  a  structure  as  a  forest  tree  an  enormous 
amount  of  material  and  of  potential  energy. 

But  this  latter,  form  of  accumulation,  devoted  especially 
to  the  production  and  maintenance  of  a  very  large  plant- 
body,  differs  materially  from  the  storage  of  a  quantity  of 
food  which  is  temporarily  a  surplus,  but  which  is  destined 
for  subsequent  consumption  by  the  protoplasts.  This  is 
a  feature  of  the  life  of  all  plants  in  varying  degrees,  whether 
they  form  a  large  plant-body  or  not.  We  must  turn  to 
examine  this  surplus  production  in  more  detail. 

In  an  earlier  chapter  we  alluded  to  the  very  marked 
division  of  labour  which  we  can  observe  in  such  a  com- 
munity of  protoplasts  as  form  a  large  plant.  We  have 
since  studied  certain  of  the  different  processes  which  are 
carried  on  by  particular  tissues  or  collections  of  protoplasts, 
rendering  them  unable  to  perform  other  necessary  duties. 
It  is  evident  that  to  enable  them  to  discharge  their  special 
functions  they  must  be  fed  and  nourished.  It  is  equally 
clear  that  they  are  not  living  under  conditions  which  enable 
them  to  construct  food  for  themselves.  We  see  that  it  is 
consequently  necessary  for  food  to  be  transported  to  them 
from  the  seat  of  its  construction. 

There  is  in  every  green  plant  a  localised,  though  fairly 
widespread,  region  in  which  construction  is  taking  place, 
and  there  are  other  equally  well-defined  regions  which 
must  be  supplied  with  food  transported  from  the  seats  of 


216  VEGETABLE  PHYSIOLOGY 

its  manufacture.  The  cell  or  protoplast,  which  contains  a 
portion  of  the  chlorophyll  apparatus,  has  thus  not  only  to 
provide  for  its  own  nutrition,  but  to  prepare  a  part  of  the 
nutritive  material  required  by  other  protoplasts  which  are 
set  apart  for  the  discharge  of  other  work. 

But  this  is  not  all.  We  find,  from  a  study  of  plants, 
that  in  almost  all  cases,  so  long  as  life  lasts,  groivth  is  proceed- 
ing. This  may  result  in  a  continuous  increase  in  the  dimen- 
sions of  the  plant-body,  or  may  lead  only  to  the  replacement 
of  parts  which  have  a  brief  existence,  and  need  to  be  renewed. 
This  is  the  case,  for  instance,  in  forest  trees  that  have 
attained  their  full  dimensions.  Growth  in  the  vegetable 
organism  is  very  definitely  localised.  Growth  in  length  takes 
place  at  or  near  the  apices  of  stems  and  roots  ;  it  has  a 
definite  though  variable  localisation  in  leaves  of  different 
kinds.  Growth  in  thickness  is  confined  to  sheaths  or  bands 
of  cells  in  different  regions  of  the  axis,  such  as  the  cambium, 
and  the  different  phellogens  met  with  in  the  cortex. 

Growth  and  nutrition  differ  in  another  respect :  the 
former  is  intermittent,  the  latter  needs  to  be  constant, 
though  the  intensity  of  the  requirements  may  vary. 

These  considerations  show  us  that  there  must  exist  in  the 
plant  a  very  complete  mechanism  by  which  the  different 
food-stuffs  can  be  circulated  about  its  body.  Each  pro- 
toplast must  be  in  receipt  of  a  continuous,  though  per- 
haps small,  supply  of  nutritive  material;  the  demands  of 
growth  must  be  satisfied  by  the  transport  of  considerable 
quantities  of  formative  material  to  the  growing  regions. 
The  intermittence  of  growth  makes  a  further  demand. 
Consider  one  among  many  places  at  which  a  large  con- 
sumption of  such  formative  material  is  proceeding  :  a  stream 
is  travelling  there  to  supply  the  need.  Suppose  that  some 
temporary  check  to  the  growth  at  that  spot  takes  place. 
The  stream  will  be  diverted  elsewhere  by  the  demands  of 
the  other  growing  parts,  and  when  the  hindrance  is  re- 
moved and  growth  should  again  proceed,  there  will  be  no 
stream  of  constructive  material,  and  much  time  will  be  lost 


TKANSLOCATION  OF  NUTRITIVE  MATERIALS     217 

before  it  can  be  restored.  To  prevent  this  there  should  be 
a  storage  of  food  close  to  the  seat  of  its  consumption,  so 
that,  with  the  awakening  need,  the  required  supply  may  be 
at  hand. 

This  temporary  storage  of  food  must  play  an  important 
part  in  the  metabolism  of  an  organ  whose  vital  processes 
are  subject  to  such  numerous  and  often  rapid  checks  as 
befall  young  stems,  leaves,  and  roots.  Still  more  necessary 
is  it  to  the  floral  .and  fruiting  organs  during  the  time  of 
their  maturing. 

We  have  seen  again  that  plants  set  apart  particular 
structures  for  periods  of  longer  quiescence,  especially  in 
connection  with  their  reproductive  processes.  Seeds  may 
remain  for  several  years  without  germinating,  and  they 
generally  do  so  at  least  for  months.  The  embryo  in  the 
seed  is,  however,  ready  to  resume  its  growth  as  soon  as  all 
conditions  are  favourable.  It  is  evident  that  it  is  in  a 
practically  helpless  condition  with  regard  to  the  manufac- 
ture of  food,  and  it  must  depend  upon  a  previously  stored 
supply  for  the  resumption  of  vital  activity.  The  parent 
plant  must,  therefore,  store  quantities  of  its  manufactured 
products  in  or  about  the  embryo  of  the  seed,  stores  with 
which  it  will  itself  have  little  further  concern,  but  which 
will  be  very  largely  the  property  of  the  new  organism.  The 
same  thing  is  seen  to  be  the  case  with  tubers,  bulbs,  and 
other  organs  of  vegetative  propagation. 

A  condition  intermediate  between  the  two  we  have  so 
far  described  is  presented  by  the  large  fleshy  roots  and 
rhizomes  of  biennial  and  perennial  plants.  For  an  illus- 
tration we  may  consider  an  ordinary  carrot  or  beetroot. 
Though  these  plants  propagate  themselves  by  the  prepara- 
tion of  flowers,  fruits,  and  seeds,  they  do  not  enter  on  this 
task  during  the  first  year  of  their  lives.  During  this  time 
they  are  in  full  foliage,  and  their  constructive  processes 
are  at  their  best.  They  store  in  their  roots  a  large  amount 
of  the  food  so  prepared,  and  these  towards  the  close  of  the 
first  year's  vegetation  become  enormously  swollen  by  the 


218  VEGETABLE  PHYSIOLOGY 

development  of  succulent  parenchyma.  During  the  second 
year  they  have  a  much  smaller  foliar  development,  but  each 
sends  up  its  flowering  stem.  The  constructive  activity  is 
much  less  than  during  the  previous  year ;  the  root  gradu- 
ally dwindles  as  the  fruit  and  seeds  develop,  the  store 
deposited  in  the  succulent  parenchyma  being  applied  to 
their  formation  and  maturity. 

Based  upon  considerations  such  as  these,  we  may  make 
a  further  classification  of  the  nutritive  substances  which 
exist  in  the  body  of  the  plant.  We  can  speak  of  those 
which  are  used  in  the  cells  where  they  are  formed,  and  of 
those  which  are  removed  therefrom  for  the  feeding  of  the 
other  protoplasts.  These,  again,  may  be  devoted  to  imme- 
diate use,  or  may  be  stored  as  reserve  materials  for  deferred 
consumption.  We  can  recognise  in  every  plant  some  kinds 
which  are  suitable  for  transport  from  cell  to  cell,  and  others 
which  are  not  able  to  pass  through  cell-walls,  but  must 
remain  in  the  position  in  which  they  are  formed.  These 
two  classes  of  circulating  and  stored  food-stuffs  have  an 
intimate  relationship  to  each  other,  and  must  be  mutually 
interdependent,  each  being  reinforced  by  the  other  accord- 
ing to  the  needs  of  the  particular  moment. 

If  now  we  turn  from  these  general  considerations  to 
the  sequence  of  events  that  are  normally  taking  place  in 
the  cells  which  contain  the  chloroplasts,  we  can  form  some 
definite  idea  of  the  course  of  the  processes  of  construction 
of  the  carbohydrates  and  removal  of  the  products.  In 
such  a  cell  there  is,  during  favourable  conditions,  a  manu- 
facture of  sugar  which  is  continuous  and  rapid.  The 
cell  itself  needs  a  certain  amount  of  such  sugar  for  its 
own  nutrition,  but  only  a  very  small  part  of  what  is  being 
made.  The  sap  of  its  vacuole  soon  contains  a  large  excess 
of  sugar,  and  if  nothing  further  transpires  the  process  of 
manufacture  must  stop.  But  the  cell  is  in  contact  with 
others,  in  many  of  which,  perhaps  in  all,  a  similar  manu- 
facture is  taking  place.  The  ordinary  processes  of  diffusion 
or  secretion  tend  to  equalise  the  amounts  in  any  contiguous 


TKANSLOCATION  OF  NUTRITIVE  MATERIALS     219 

cells,  so  that  very  soon  the  whole  of  the  parenchyma  of  the 
constructive  region  is  plentifully  supplied  with  the  sugar. 
This  parenchyma  abuts,  however,  on  other  cells  which  con- 
tain no  chloroplasts,  especially  the  sheaths  and  the  bast  of 
the  fibro- vascular  bundles.  Diffusion  of  sugar  into  these 
takes  place,  and  proceeds  from  cell  to  cell,  especially  among 
the  delicate  bast  tissue,  so  that  a  stream  of  sugar  is  soon  dif- 
fusing all  along  the  bast.  As  a  rule  it  does  not  penetrate 
very  far  beyond  this  tissue,  owing  largely  to  the  anatomical 
arrangements  of  the  parts  and  the  great  facility  which  the 
structure  of  the  bast  affords  for  this  diffusion.  So  long  as 
the  manufacture  goes  on,  therefore,  there  is  an  outflow  of 
the  manufactured  carbohydrates  from  the  region  of  its  forma- 
tion, the  ultimate  and  even  the  temporary  direction  of  the 
stream  being  determined  by  other  factors  which  we  shall 
consider  later. 

This  removal  of  sugar  from  the  leaf  can  be  proved  by 
several  observations.  We  find  but  little  of  it  in  the  meso- 
phyll  of  the  leaf,  though  we  know  it  is  being  continually 
produced  there.  We  find  it  fairly  easily  in  the  bast  of  the 
veins,  and  if  a  leaf  is  cut  off  from  the  stem  while  construc- 
tion is  going  on,  so  that  it  cannot  be  transported  away,  it 
can  very  soon  be  detected  in  the  mesophyll  cells  as  well. 

This,  however,  is  not  all.  The  process  of  translocation  is 
a  slow  one  and  does  not  serve  to  remove  the  sugar  as  fast 
as  it  is  formed.  The  excessive  formation  of  sugar  would 
soon  lead  to  such  a  saturation  of  the  sap  as  would  at  any 
rate  temporarily  inhibit  its  construction,  were  it  not  for 
another  agency  at  work.  The  chloroplasts  are  endowed 
with  another  property  than  that  so  far  described,  which  is 
now  called  into  play.  This  is  a  peculiarity  of  the  body  of 
the  plastid,  and  is  quite  independent  of  the  colouring 
matter,  being  shared  by  other  quite  colourless  plastids 
which  occur  in  other  parts  of  the  plant.  These  structures 
have  the  power  of  converting  sugar  into  starch,  a  power 
which  we  must  examine  more  fully  in  a  subsequent  chapter. 
The  transformation  is  apparently  a  process  of  secretion. 


220  VEGETABLE  PHYSIOLOGY 

Part  of  the  sugar  consequently  gives  rise  to  numerous 
minute  grains  of  starch,  which  the  plastid  forms  within  itself 
and  deposits  in  its  own  substance.  This  formation  of  a  tem- 
porary store  of  starch  not  only  relieves  the  over-saturation 
of  the  sap  in  the  cell,  but  supplies  the  need  of  the  protoplasm 
when  the  formation  of  sugar  from  carbon  dioxide  and  water 
is  interrupted  by  the  failure  of  the  daylight,  being  then  re- 
converted into  sugar.  These  minute  granules  are  of  very 
small  dimensions,  three  or  four  of  them  being  formed  within 
each  plastid.  They  have  no  apparent  structure,  but  can  be 
detected  by  treating  the  cell  with  a  solution  of  iodine, 
which  stains  them  blue.  If  a  chloroplast  so  treated  is 

examined  with  a  high  power  of  the  micro- 
lUj  scope,  it  presents  the  appearance  of  fig.  104, 

the  little  grains  of  starch  lying  as  blue  specks 

in  the  green  substance.  They  can  be  seen 
FIG.  104.— STARCH  more  distinctly  if  the  leaf  under  examination 
BODIES  OFNC™OE  is  bleached  by  warming  it  in  alcohol,  which 
KOPLASTS.X250.  dissolves  out  the  chlorophyll.  A  leaf  so 

treated  turns  blue  wherever  the  light  has  had 
access  to  it,  not  only  showing  the  formation  of  the  starch, 
but  allowing  its  exact  locality  to  be  determined  with 
absolute  precision.  In  fact,  this  test  may  be  applied 
to  ascertain  whether  the  chlorophyll  apparatus  of  a 
part  is  at  any  time  active,  the  deposition  of  the  starch 
taking  place  within  a  few  minutes  of  the  commencement 
of  carbohydrate  construction.  This  rapidity  of  appear- 
ance led  indeed  to  the  old  view  that  the  construction  of 
starch  rather  than  sugar  was  the  immediate  object  of  the 
chlorophyll  apparatus.  The  reasons  we  have  given  lead  us 
preferably  to  the  view  that  the  starch  is  the  expression  of  the 
superabundant  supply,  requiring  that  a  certain  portion  shall 
be  deposited  in  an  insoluble  form  as  a  temporary  reserve 
material,  to  allow  the  process  of  carbohydrate  construction 
to  proceed  without  intermission  so  long  as  the  conditions 
are  favourable.  At  the  same  time  we  cannot  but  notice 
that  the  appearance  of  starch  in  the  chloroplasts  is  so  rapid 


TBANSLOCATION  OF  NUTEITIVE  MATEKIALS     221 

when  the  conditions  of  carbohydrate  formation  are  realised, 
that  it  may  be  relied  on  as  a  test  for  the  absorption  of  carbon 
dioxide  by  the  tissue  in  which  it  appears. 

In  connection  with  the  manufacture  and  fate  of  carbo- 
hydrates, we  can  now  see  that  they  may  be  met  with  in 
two  different  conditions  :  the  one  suitable  for  retention  in 
the  cell  and  hence  capable  of  functioning  as  reserve,  but 
not  immediately  nutritive,  material :  the  other  capable  of 
diffusion,  and  hence  serving  as  a  translocatory  form,  or 
one  in  which  it  can  pass  from  cell  to  cell,  remaining  all 
the  time  in  a  suitable  condition  to  minister  to  the  nutrition 
of  any  protoplasm  which  it  reaches. 

The  same  considerations  affect  the  manufacture,  trans- 
port, and  storage  of  proteins.  We  have  already  seen 
reason  to  believe  that  these,  like  the  carbohydrates,  are  in 
the  first  instance  constructed  in  the  leaves,  if  not  by  the 
chloroplasts.  Our  information  about  them  is,  however, 
very  incomplete  ;  we  do  not  know  even  what  form  of 
protein  is  first  formed,  nor  which  kind  is  needed  for 
assimilation  by  the  protoplasm.  Possibly  it  is  a  soluble 
and  diffusible  form,  such  as  a  peptone  or  a  proteose,  but 
our  only  reason  for  thinking  so  is  that  such  properties 
characterise  the  travelling  forms  of  carbohydrates.  We 
can,  however,  readily  believe  in  the  construction  being 
greatly  in  excess  of  the  immediate  need  of  the  cell,  and 
hence  in  the  chain  of  events  being  similar  to  that  in  which 
the  carbohydrates  are  concerned. 

The  different  properties  of  the  two  classes  of  bodies 
involve,  however,  some  differences  in  their  behaviour, 
and  we  can  therefore  expect  similarity  only  and  not  iden- 
tity. The  diffusibility  of  peptone,  even,  is  very  greatly 
below  that  of  sugar  ;  and  we  can  hardly  suppose  therefore 
that  peptone  is  the  translocatory  form  of  protein  in  the 
plant.  It  seems  more  probable  that  nitrogenous  plastic 
material  is  transported  in  the  form  of  some  amino-  or 
amido-acid  such  as  asparagin.  This  view  is  supported 
by  observations  made  upon  the  utilisation  of  the  reserve 


222  VEGETABLE    PHYSIOLOGY 

stores  of  proteins  found  in  seeds,  which  have  been  found 
to  give  rise  to  similar  amino-acids  before  being  transported 
from  the  site  of  storage.  To  this  point  we  shall  return  in 
a  subsequent  chapter. 

We  cannot  say  either  in  what  form  proteins  are  tem- 
porarily stored  in  the  cells  of  their  first  formation.  Pro- 
bably, like  starch,  they  are  made  indiffusible  and  so  retained 
in  the  cell.  But  whether  they  are  thrown  into  a  solid 
form  we  do  not  know.  If  so,  they  are  amorphous  and  are 
hidden  away  in  the  substance  of  the  protoplasm.  They  may 
be  kept  in  solution  in  the  sap  which  saturates  it.  Different 
kinds  of  globulin  and  albumin  have  been  found  in  the 
cells  in  different  regions.  It  is  possible  again  that  the 
manufacture  of  protein  may  be  only  so  great  as  to  provide 
for  the  needs  of  the  cells  in  which  such  formation  takes 
place,  together  with  the  amount  that  can  diffuse  during 
such  manufacture,  so  that  there  may  be  no  occasion  for  a 
temporary  storage  there.  We  are  not  sure  whether  the 
process  of  their  formation  is  a  continuous  one,  or  is  inhibited 
at  night. 

The  translocation  of  food  has  no  very  determinate 
direction.  On  leaving  the  cells  which  are  the  seats  of  its 
formation,  its  path  is  dependent  on  physical  processes 
taking  place  in  different  parts  of  the  plant.  We  can  study 
it  most  simply  by  taking  a  special  case,  which  as  before 
may  conveniently  be  that  of  sugar.  We  are  not  familiar 
with  the  physical  process  of  its  passage  from  cell  to  cell : 
it  is  unlikely  that  osmosis  causes  it  to  pass  so  regularly,  and 
diffusion  is  apparently  not  sufficiently  rapid  to  explain  it  : 
possibly  it  may  be  picked  out  from  the  cell-sap  by  the  proto- 
plasm and  passed  on  to  the  vacuole  of  the  next  cell  and 
so  forward  by  a  kind  of  secretion.  We  know,  however, 
that  it  is  conducted  through  the  parenchyma  to  the 
nbro-vascular  bundles,  the  bast  of  which,  we  have  seen, 
forms  its  principal  path.  These  extend  in  complete  con- 
tinuity throughout  the  plant,  so  that  any  travelling  com- 
pound can  be  transported  from  the  leaves  to  the  growing 


TBANSLOCATION  OF  NUTKITIVE  MATEEIALS     223 

points  of  the  stem  and  root.  So  long  as  it  is  being  used 
by  the  protoplasm  in  these  regions,  the  sap  of  the  cells  of 
the  tissue  there,  which  are  using  it  in  the  construction  of 
living  substance,  becomes  continually  weaker  in  that  con- 
stituent, and  hence  more  and  more  makes  its  way  into  them 
to  equalise  the  concentration.  The  utilisation  or  consump- 
tion of  the  sugar  so  acts  as  an  attracting  force,  directing 
the  stream  to  the  points  where  it  is  required.  The  same 
principle  applies  to  the  consideration  of  the  deposition  of 
the  large  reserves  of  carbohydrates  in  seeds,  tubers,  or 
other  organs.  The  withdrawal  of  it  from  the  travelling 
stream,  which  is  the  result  of  the  formation  of  the  quantities 
of  starch  or  cellulose  which  those  reservoirs  contain,  leads 
to  fresh  quantities  being  transported  slowly  but  con- 
tinuously to  those  cells,  owing  to  the  same  physical  pro- 
cesses. The  stream  passes,  in  fact,  in  both  cases  exactly  in 
proportion  as  the  consumption  takes  place,  whether  the 
consumption  takes  the  form  of  construction  of  new  proto- 
plasm, or  the  transformation  of  the  travelling  carbohydrates 
into  the  insoluble  resting  forms. 

This  passage  of  the  sugar  about  the  plant  need  not  demand 
a  coincident  transport  of  water,  so  that  the  old  idea  that 
there  was  an  actual  stream  of  fluid  along  the  bast,  or  in 
the  old  nomenclature  a  stream  of  descending  sap,  need  not 
have  any  foundation  in  fact.  The  principle  of  diffusion 
and  the  action  of  the  protoplasm  will  explain  the  passage 
of  the  sugar.  Disturbances  of  the  fluid  contents  of  the 
cells  do  no  doubt  occur,  as  osmosis  is  continually  taking 
place  in  both  directions  between  the  contiguous  cells.  A  defi- 
nite flow  of  water  need  not,  however,  coincide  in  either  mag- 
nitude or  direction  with  the  passage  of  the  stream  of  sugar. 

The  translocation  of  the  sugar,  we  see,  thus  varies  in 
direction  and  in  magnitude  according  to  the  varying  pro- 
cesses which  are  from  time  to  time  proceeding.  As  the 
variations  in  these  processes,  particularly  those  of  growth 
and  nutrition,  are  often  sudden  and  considerable,  we  find 
the  translocation  is  generally  accompanied  by  changes  of 


224  VEGETABLE  PHYSIOLOGY 

the  carbohydrate  from  the  travelling  to  the  storage  forms, 
and  vice  versa.  It  is  very  usual  to  find  temporary  accumula- 
tions- of  starch  in  the  neighbourhood  of  a  growing  region. 
Grains  of  starch  are  of  frequent  occurrence  in  different 
parts  of  the  bast,  and  particularly  in  the  bundle-sheaths  of 
certain  regions.  The  explanation  of  their  appearance  there 
is  simple ;  they  are  generally  indications  of  such  an  inter- 
ference with  the  supply  and  the  demand  as  we  have  described. 
A  checking  of  the  demand  by  a  cessation  of  the  vigour  of 
growth  or  nutrition  is  attended  by  an  over-accumulation 
of  the  sugar,  which  is  speedily  changed  into  a  storage  form. 

The  transport  of  proteins  follows  the  same  course  ;  the 
amino-  or  amido-acids  are  the  travelling  forms,  and  are 
conducted  by  the  same  forces  to  the  growing  points,  or  to 
reservoirs  where  accumulation  of  proteins  takes  place. 
Their  deposition  in  storage  forms  along  the  pathway  can 
also  be  detected,  though  these  are  not  so  widespread  as 
those  of  carbohydrates.  They  can  be  observed  generally  in 
the  sieve-tubes  of  the  bast,  which  contain  a  curious  modifica- 
tion of  protoplasm  in  which  protein  as  such  is  present.  It 
was  formerly  held  that  the  sieve-tubes  conduct  protein  as 
such  along  the  vascular  bundles.  Though  there  is  not  a 
very  great  improbability  that  such  bodies  may  pass  from 
cell  to  cell  of  the  sieve-tube,  on  account  of  the  protoplasmic 
or  quasi-protoplasmic  threads  which  extend  throughout  the 
openings  of  the  sieve-plates,  yet  this  method  of  transport 
must  be  necessarily  very  slow  and  subject  to  much  hindrance. 
It  seems  more  probable  that  the  proteins  in  these  vessels 
are  constructed  there  from  the  amino-acids  which  reach 
them,  and  are  to  be  regarded  as  temporary  stores,  like  the 
starch  grains  already  alluded  to  as  being  formed  in  different 
parts  of  the  translocatory  tract. 

We  have  spoken  of  the  bast  as  forming  the  pathway  of 
the  translocation  of  nutritive  material  or  of  the  different 
food-stuffs  which  have  been  manufactured.  The  protoplas- 
mic threads  that  extend  through  the  openings  of  the  sieve- 
plates  no  doubt  afford  facilities  of  passage.  It  must  not 


TEANSLOCATION  OF  NUTEITIVE  MATEKIALS    225 

be  forgotten,  however,  that  all   cell-membranes  are  per- 
forated by  very  delicate  strands  of  protoplasm  which  extend 
from  one  protoplast  to  another.     There  is  here  a  path  of- 
transport  by  which  the  protoplasm  can  pass  substances  from 
cell  to  cell  as  already  suggested. 

We  may  find  proofs  that  the  pathway  lies  along  the  bast 
by  experiment  carried  out  on  plants  in  which  translocation 
is  actively  proceeding.  If  we  cut  a  branch  from  a  suitable 
vigorously  growing  tree  and  remove  from  near  its  free  end 
a  ring  of  tissue  extending  inwards  through  the  bark  and 
cortex  to  the  cambium,  and  then  place  all  the  lower  part  in 
water  or  moist  earth,  very  marked  effects  follow.  After 
some  time,  perhaps  a  few  weeks,  adventitious  roots  will  be 
put  out  from  near  its  end.  Those  which  arise  below  the 
missing  ring  will  be  few  and  of  small  size  ;  those  from  above 
this  region  will  be  numerous  and  strong,  and  will  continue 
to  elongate.  Any  buds  that  may  be  on  the  part  below  the 
ring  will  not  develop,  while  those  above  it  will  grow  normally 
or  even  more  freely  than  on  an  uninjured  branch.  The 
tissue  immediately  above  the  ring  will  become  somewhat 
hypertrophied  and  show  a  decided  swelling. 

The  continuance  of  the  growth  shows  that  the  water 
supply  has  not  been  cut  off,  but  the  different  behaviour  of 
the  parts  above  and  below  the  excised  tissue  tells  us  that 
the  supply  of  nutritive  material  to  the  latter  region  has 
been  interfered  with,  and  the  buds  and  adventitious  roots 
it  bears  gradually  perish  of  inanition.  The  passage  of 
any  food  or  nutritive  material  across  the  ring  has  become 
impossible. 

If  a  similar  incision  is  made  into  another  branch,  but  is 
not  carried  so  far  inwards — if,  that  is,  the  ring  of  tissue 
removed  consists  only  of  the  structures  external  to  the 
bast — these  appearances  do  not  accompany  or  follow  the 
wound.  Evidently  in  this  case  the  translocation  path  has 
not  been  interfered  with.  We  may  safely  conclude  there- 
fore that  the  transport  of  elaborated  products,  chiefly  food, 
is  the  principal  function  of  the  bast. 

15 


226  VEGETABLE  PHYSIOLOGY 

To  a  certain  extent  the  cortex  of  the  plant  shares  the 
translocatory  function.  The  contents  of  its  cells  include  a 
certain  amount  of  carbohydrate  material,  but  their  reaction 
is  distinctly  acid,  so  that  this  region  is  probably  concerned 
much  more  definitely  with  the  transport  of  vegetable  acids, 
so  far  as  it  takes  part  in  translocation  at  all.  At  the  same 
time  it  is  impossible  to  localise  the  transport  of  food 
exclusively  in  the  bast. 

Other  parenchymatous  tissues  are  sometimes  the  region 
of  transport.  In  many  germinating  seeds  there  is  a  trans- 
ference of  large  quantities  of  nutritive  substance  across  the 
endosperm  to  the  embryo,  and  in  young  seedlings  similar 
transport  takes  place  through  pith  as  well  as  cortex. 

The  vessels  of  the  wood,  which  we  have  seen  are  the  paths 
of  the  transpiration  current  are  probably  not  concerned 
normally  in  the  translocation  of  manufactured  products, 
though  exceptionally  they  may  contain  certain  amounts 
of  proteins,  amido-acids,  &c.,  in  solution.  Their  function 
in  this  respect  is,  however,  unimportant,  and  the  presence 
of  such  bodies  in  them  is  mainly  accidental. 

An  important  exception  is  seen  in  the  resumption  of  the 
growth  of  a  tree  in  the  spring.  Before  the  unfolding  of  its 
leaves  enables  photosynthesis  to  commence,  a  stream  of  food 
from  the  winter  reservoirs  takes  place  which  travels  upwards 
in  the  wood. 

It  is  doubtful  how  far  the  laticiferous  systems,  which 
are  present  in  many  plants,  may  be  regarded  as  channels 
for  translocation.  No  doubt  latex  contains  many  nutritive 
products,  both  nitrogenous  and  non-nitrogenous,  but  there 
is  reason  to  think  they  are  to  be  referred  to  the  storage 
rather  than  to  the  transporting  system. 


227 


CHAPTEK  XV 

THE    STORAGE    OF   RESERVE    MATERIALS 

We  have  seen  that  the  large  amount  of  food  which  is 
continually  being  manufactured  by  a  normal  green  plant  is 
very  greatly  in  excess  of  its  immediate  requirements,  and 
that  there  is  a  very  extensive  system  of  storage  in  such  an 
organism,  by  the  aid  of  which  it  is  enabled  to  survive 
periods,  often  of  some  duration,  in  which  the  manufacturing 
processes  are  entirely  suspended.  We  have  considered 
further  the  mechanisms  of  transport,  by  which  the  various 
nutritive  substances  are  transferred  from  the  seats  of  their 
manufacture  to  the  places  in  which  they  are  laid  up  for 
longer  or  shorter  periods. 

The  questions  of  transport  and  of  storage  are  very  inti- 
mately connected.  Food  once  formed  is  not  always  moved 
at  once  to  some  place  where,  after  a  period  of  storage,  it 
will  be  ultimately  consumed.  It  is  often  transferred  more 
than  once,  and  may  occupy  several  places  in  succession 
as  the  demand  for  it  varies.  Indeed,  we  may  regard  the 
surplus  manufactured  food,  that  is  the  quantity  which  is 
in  excess  of  the  immediate  requirements  of  the  constructive 
cells,  as  a  single  store,  part  of  which  is  travelling  about  the 
plant,  and  part  of  which  is  from  time  to  time  withdrawn 
from  the  travelling  stream  and  laid  down  in  particular 
cells,  either  to  rejoin  the  travelling  current  after  a  longer 
or  shorter  time,  or  to  be  separated  from  the  parent  plant, 
and  serve  as  a  starting  point  for  the  growth  and  nutrition 
of  its  offspring. 

15* 


228  VEGETABLE  PHYSIOLOGY 

A  very  little  consideration  will  show  us  that  the  forms 
in  which  the  various  food-stuffs  are  packed  away  in  the 
storage  reservoirs  must  be  materially  different  from  those 
in  which  they  travel.  We  have  already  seen  that  one  of 
the  conditions  of  the  continuous  formation  of  any  one  of 
them  is  the  removal  of  it  from  the  seat  of  its  construction  as 
soon  as  its  amount  exceeds  a  certain  limit.  If  this  is  not 
secured,  the  sap  of  the  constructing  cells  soon  contains  as 
much  of  the  body  in  question  as  it  will  hold,  and  then  no 
more  is  made.  The  removal  is  dependent  upon  the  depo- 
sition of  the  substance  from  the  sap  in  some  way  which 
lessens  the  concentration  of  its  solution  in  the  latter.  We 
find  accordingly  that  the  bulkier  reserve  materials  are  very 
frequently  deposited  in  solid  forms,  sometimes  amorphous, 
sometimes  granular,  and  sometimes  crystalline.  Other 
cases  are  known  as  well,  in  which  they  remain  in  solution 
in  the  sap  of  particular  cells,  but  in  these  cases  they  are 
retained  in  such  cells  through  the  difficulty  or  impossibility 
of  passing  through  the  cytoplasm.  They  are  generally 
formed  inside  these  cells  from  some  particular  constituent 
of  the  travelling  stream,  much  as  are  those  which  become 
insoluble,  and  once  formed,  they  are  unable  to  pass  out  of 
the  vacuole. 

In  considering  the  forms  which  the  various  reserve  food 
materials  assume  in  the  reservoirs  they  occupy,  we  must 
then  remember  that  they  are  not  a  simple  accumulation  of 
food  pabulum  in  the  form  in  which  it  is  of  immediate  use. 
Granted  that  the  plant  in  the  first  instance  forms  certain 
materials  on  which  its  living  substance  draws  at  the  place 
where  it  is  originally  constructed,  then,  so  long  as  the 
immediate  needs  are  in  excess  of  the  amount  prepared, 
there  is  no  alteration  in  such  materials  ;  they  are  at  once 
utilised  by  the  living  substance  in  the  processes  of  nutrition 
and  growth.  But  as  soon  as  the  supply  exceeds  the  imme- 
diate demand,  the  surplus  is  not  simply  retained  unchanged 
in  the  cell,  nor  does  it  overflow  unchanged  to  contiguous 
cells  where  demand  exceeds  supply,  or  where  provision  is 


THE  STOBAGE  OF  EESEKVE  MATEBIALS      229 

made  for  storage.  The  storage  forms,  whether  retained  in 
the  cells  of  construction  or  transferred  to  others,  are  different 
from  and  more  complex  than  the  originally  prepared  ones, 
and  further  energy  has  to  be  expended  on  them,  either 
where  they  are  made,  or  in  the  place  of  storage  itself. 

As  we  shall  see  later,  when  they  come  to  be  utilised  in 
after  time,  a  converse  process  takes  place,  which  is  com- 
parable to  the  digestion  which  they  undergo  when,  as  so 
frequently  happens,  they  are  eaten  by  an  animal. 

The  surplus  food  of  the  plant  exists  thus  in  two  conditions, 
the  one  suitable  for  travelling,  the  other  for  storage.  The 
former  is  characterised  by  solubility  and  diffusibility,  the 
latter  generally  by  insolubility  in  the  cell-sap,  and  always 
by  an  absence  of  the  power  to  pass  through  the  protoplasmic 
membranes.  The  former  usually  consists  of  such  substances 
as  can  at  once  be  assimilated  by  the  living  material;  the 
latter  does  not,  but  requires  the  digestive  changes  to  take 
place  before  it  becomes  so. 

The  places  where  these  reserve  materials  are  deposited 
are  more  numerous  than  we  are  apt  to  suppose.  Parts  of 
the  plant,  or  definite  structures  which  ultimately  serve  as 
reproductive  organs,  readily  occur  to  us  as  reservoirs  which 
are  adapted  for  a  somewhat  prolonged  storage.  Seeds, 
tubers,  fleshy  roots  and  branches,  bulbs,  corms,  and  rhizomes 
•  are  instances  of  these,  and  in  the  short-lived  plants  which  we 
group  together  roughly  as  herbaceous  in  their  habit,  these  are 
necessarily  the  most  important  reservoirs.  But  it  is  different 
with  trees  and  shrubs  which  live  for  many  years,  and  which 
do  not  form  fleshy  receptacles.  We  have  in  these  forms 
stout  stems  or  trunks,  with  numerous  branches ;  large 
woody  roots  which  continue  to  grow  year  after  year,  keep- 
ing pace  with  the  parts  above  ground.  Though  the  primary 
use  of  these  members  is  not  to  store  food  products,  yet  they 
have  work  of  this  kind  to  do.  We  have  seen  that  in  the 
cells  which  are  the  original  seats  of  carbohydrate  construc- 
tion there  is  almost  always  an  excess  of  such  matter  formed, 
which  is  partly  deposited  in  the  chloroplasts  in  the  form 


230  VEGETABLE  PHYSIOLOGY 

of  small  granules  of  starch.  These  afford  us  an  instance 
of  a  very  transitory  store,  for  the  starch  deposited  there 
during  exposure  to  sunlight  is  removed  almost  as  soon  as 
darkness  supervenes.  A  plant  which  has  been  vigorously 
forming  starch  in  its  chloroplasts  during  a  summer's  day 
will  show  that  at  evening  there  is  a  considerable  amount 
accumulated  there  ;  if  the  leaves  are  examined  again  early 
next  morning,  the  starch  will  be  found  to  have  disappeared. 
This  is  not  brought  about  by  its  having  been  used  in  the 
metabolism  of  the  cells  during  the  .night,  for  if  the  path  of 
removal  is  obliterated,  as  it  may  be  by  severing  the  petiole 
in  the  evening,  the  leaf  is  found  as  full  as  ever  in  the  morn- 
ing. If  a  plant  whose  chloroplasts  are  charged  with  starch 
grains  is  kept  for  a  time  in  an  atmosphere  free  from  carbon 
dioxide,  the  starch  is  gradually  removed,  whether  it  is  kept 
in  light  or  darkness,  so  that  the  removal  of  the  starch  can; 
and  probably  does,  take  place  continuously,  though  it 
cannot  be  easily  detected  so  long  as  construction  is  proceed- 
ing simultaneously. 

The  deposition  of  food  in  such  other  reservoirs  in  trees 
and  shrubs  as  are  not  connected  with  the  reproduction  of 
the  plant  is  generally  of  a  transitory  character,  though  not 
so  markedly  so  as  in  the  case  of  the  leaves.  These  temporary 
storage  places  pre  found  very  widely  distributed,  and  the 
reason  for  their  occurrence  is  in  each  case  traceable  with 
comparative  ease.  A  tree  that  has  a  trunk  arid  a  root 
which  are  growing  in  thickness  is  in  need  of  a  constant 
rather  than  an  intermittent  supply  of  food  placed  near  the 
actively  growing  regions.  The  growth  in  thickness  of  such 
a  trunk  or  root  is  brought  about  by  the  activity  of  a  layer 
of  delicate  living  cells,  which  are  constantly  dividing  to 
produce  new  wood  and  new  bast,  and  which  appear  quite 
early  as  a  ring  of  cambium  on  the  exterior  of  the  woody 
mass  (fig.  105,  fe).  The  new  cells  need  a  constant  supply  of 
nutritive  material,  at  the  expense  of  which  they  develop 
into  the  peculiar  elements  of  wood  and  bast  respectively. 
The  cambium,  too,  is  in  continuous  need  of  food,  or  it  is 


THE  STORAGE  OF  RESERVE  MATERIALS      281 

perforce  obliged  to  cease  dividing,  and  so  the  growth  in 
thickness  of  the  trunk  or  root  is  stopped.  Cell-division  is 
indeed  the  result  of  cell-growth.  When  a  cell  of  the  cambium 
has  attained  its  full  size  it  divides  into  two,  each  of  which 
then  grows  to  its  appropriate  adult  dimensions  ;  some 
divide  again,  like  those  from  which  they  sprang  ;  others 
become  transformed  into  wood  or  bast  cells.  In  either 
case  an  immediate  supply  of  food  is  needed,  and  from  the 
condition  of  things  this  must  be  near  at  hand.  The  stream 


FIG.  105. — SECTION  OF  PART  OF  STEM  OF  Eicinua  communis. 

a,  starch  sheath ;   at  the  extremities  of  the  figure  its  cells  are 
represented  as  empty ;  6,  cambium  layer. 

from  the  leaves  is  intermittent,  and  hence  it  is  important 
that  a  certain  reserve  shall  be  deposited  not  far  from  the 
growing  cells,  so  that  a  slow  continuous  supply  may  be 
available.  We  find  such  reserves  laid  down  near  the 
cambium,  either  in  the  cells  of  definite  sheaths  surrounding 
the  whole  ring  of  new  tissue  (fig.  105,  a),  or  in  the  spaces 
called  medullary  rays,  which  are  found  between  the  separate 
masses  of  wood  and  bast,  these  rays  (fig.  106)  being  com- 
posed of  cells  which  differ  in  shape  from  the  typical  forms 
of  both  wood  and  bast  cells. 
In  stems  of  smaller  girth  which  have  not  developed  much 


232 


VEGETABLE  PHYSIOLOGY 


wood,  we  find  stores  of  food  laid  up  in  the  region  just  under- 
neath the  surface,  which  constitutes  what  is  called  the 
cortex,  and  which  gives  place  later  on  to  the  complex  forma- 
tion that  is  familiar  to  us  under  the  name  of  bark. 

The  formation  of  the  successive  rings  of  cork  deeper  and 
deeper  in  the  cortex,  which  ultimately  constitute  the  bark, 


m.r: 


FIG.  106. — SECTION   OF    THREE-YEAR-OLD   STEM    OF  Tilia,  SHOWING   THE 
MEDULLARY  RAYS  RUNNING  THROUGH  THE  WOOD,    x  60.    (After  Kny.) 

is  attended  by  the  same  need  of  a  continuous  instead  of  an 
intermittent  supply  of  food.  We  find,  therefore,  during 
the  process  of  the  construction  of  the  bark,  similar  pro- 
vision of  food-containing  tissue,  which  is  situated  near  the 
cork  layers.  In  some  cases  it  takes  the  form  of  regular 
sheaths  ;  in  others  the  food  is  irregularly  distributed  through 
the  cortex,  which  is  the  seat  of  the  appearance  of  the  forma- 
tive layers  of  the  cork. 


THE  STOEAGE  OF  EESEEVE  MATERIALS     233 

As  the  trunk  grows  older  similar  stores  of  food  may  be 
detected  deeper  in  the  wood.  These  generally  occur  in 
the  medullary  rays,  either  those  which  are  the  continua- 
tions of  the  primary  ones,  or  others  which  are  formed 
apparently  for  the  purpose  under  discussion.  These  stores 
are  especially  for  the  nutrition  of  the  more  deeply  placed 
wood-cells,  when  the  ordinary  constructive  processes  are 
in  abeyance,  as  in  the  winter-time. 

Transitory  stores  may  also  be  detected  near  the  growing 
points  of  the  axis.  These  are  due  to  intermission  of  growth 
and  a  consequent  sudden  cessation  of  the  demand  upon  the 
translocation  stream.  The  latter,  instead  of  being  diverted 
at  once  from  the  region  to  which  it  had  been  travelling, 
deposits  in  a  suitably  stable  form  the  food  which  would 
have  been  consumed  had  not  the  check  in  the  demand 
occurred.  The  supply  is  consequently  ready  to  hand  as 
soon  as  growth  sets  in  again. 

Deposits  of  reserve  materials  can  be  observed  near  the 
extremities  of  twigs  as  winter  approaches.  The  output  of 
the  young  leaves  in  the  spring  is  greatly  facilitated  by  the 
occurrence  of  such  temporary  storage.  It  is  possible  by 
appropriate  pruning  to  influence  to  a  considerable  extent 
the  locality  and  the  extent  of  such  deposition.  This  is  of 
very  common  occurrence  in  horticulture,  the  nature  of  the 
pruning  having  in  this  way  a  very  considerable  influence 
upon  the  development  of  floral  or  foliage  shoots. 

Transitory  deposits  of  food  take  place  also  in  the  floral 
organs.  In  many  flowers  which  have  long  succulent  styles, 
which  must  be  perforated  by  the  pollen  tubes  on  their  way 
to  the  ovules,  there  may  be  observed  very  frequently  a 
deposition  of  food  in  the  tissue  of  the  style  at  the  time 
when  the  germination  of  the  pollen  grain  takes  place  upon 
the  stigma.  The  food  is  then  usually  stored  in  the  paren- 
chymatous  tissue  which  surrounds  the  vascular  bundles  of 
the  organ. 

Many  of  these  reservoirs  show  by  their  structure  that 
they  are  only  intended  to  compensate  for  regular  or 


234  VEGETABLE  PHYSIOLOGY 

accidental  intermittence  in  the  translocatory  stream  to  the 
parts  in  question.  The  food  is  temporarily  stored  in  the 
ordinary  parenchymatous  cells  or  in  the  sheaths  of  the  con- 
ducting tissue,  and  no  special  arrangements  are  made  to 
receive  it.  It  is  often  of  accidental  occurrence — deposited 
suddenly  and  gradually  or  rapidly  removed.  Such  deposi- 
tion and  re-absorption  form,  indeed,  one  of  the  features  of 
the  transporting  mechanisms. 

We  may  now  pass  to  the  consideration  of  the  forms  in 
which  the  different  foods  present  themselves  in  these  re- 
servoirs of  storage.  It  is  not  surprising  that  we  find  here 
a  great  deal  of  variety,  even  in  any  particular  class  of  food. 
The  more  prolonged  the  stay  in  the  reservoir,  the  more 
complex  usually  is  the  structure  which  the  nutritive 
substance  assumes. 

We  may  deal,  in  the  first  instance,  with  the  stores  of 
carbohydrates.     We    have    already    noticed    that    in    the 
great  majority  of  cases  these  take  the  form  of  starch.     In 
the   chloroplasts    in   the   leaf-cells    the   starch   grains   are 
laid  down  as  minute  bodies,  showing  hardly 
A  any  trace  of  structure  and  crowded  together 

in  the  substance  of  the  plastid  till  they  are 
almost  in  contact  with  each  other  (fig.  107). 
FIG.  107.— STABCH    The  deposition  is  due  to  the  protoplasm  or 
BODIES  O™CHLO.    stroma  of  the  plastid,  and  does  not  depend 
ROPLASTS.  x  250.    in  any  wav  upon  the  colouring  matter,  the 
presence  of  the  latter  influencing  only  the 
other  function  of  the  chloroplast,  the  synthesis  of  sugar, 
as  we  have  already  seen  in  a  previous  chapter.     The  process 
is  thus  one  of  true  secretion,  and  the  deposition  of  the 
starch  originating  at  several  centres  in  the  plastid,  several 
granules  are  coincidently  formed.     The  number,  however, 
is  not  constant. 

In  the  more  permanent  reservoirs  of  starch  it  usually 
happens  that  the  cells  are  so  charged  with  the  grains  that 
they  appear  to  contain  nothing  else.  Fig.  108  shows  a 
cell  taken  from  the  interior  of  a  potato  tuber.  These 


THE  STOEAGE  OF  EESEKVE  MATERIALS      235 

grains  of  starch  are  much  larger  than  those  which  occur 
in  the  chloroplasts  of  the  leaf,  and  they  have  a  complicated 
structure.  Most  of  them  are  irregularly  oval  in  shape, 
and  their  surfaces  are  marked  by  nearly  concentric  lines" 
of  striation,  dividing  them  apparently  into  layers.  The 
centre  of  these  layers  is  not  usually  the  geometrical  centre 
of  the  grain,  but  lies  near  the  small  end,  and  the  rings  or 
layers  are  much  narrower  at  that  end  than  at  the  other 
(%.  109). 

In  most  cases  the  deposition  of  starch  in  these  and  similar 
cells  is  brought  about  by  the  agency  of  small  protoplasmic 
corpuscles,  which  closely  resemble  the  chloroplasts,  except 
that  they  are  colourless.  They  are  known  for  this  reason 


FIG.  108. — CELL  OF  POTATO  FIG.  109. — STARCH  GRAIN 

CONTAINING  STARCH  GRAINS.  OF  POTATO. 


as  leucoplasts  ;  like  the  chloroplasts,  they  occur  in  con- 
siderable numbers  in  each  cell,  being  situated  usually  near 
the  nucleus.  Their  relationship  to  chloroplasts  is  shown 
by  the  fact  that  they  turn  green  when  they  are  exposed  for 
a  considerable  time  to  light. 

The  leucoplasts  behave  very  much  like  the  chloroplasts. 
When  a  solution  of  sugar  reaches  the  cell  in  which  they 
lie,  they  absorb  it  as  the  chloroplasts  do  the  excess  of  sugar 
manufactured  in  the  cells  of  the  leaf.  They  then  secrete 
starch,  which  is  at  once  deposited  in  their  substance.  If 
the  point  of  deposition  is  the  centre  of  the  leucoplast, 
successive  shells  of  starch  are  deposited  concentrically  upon 
the  first-formed  portion,  and  a  symmetrical  grain  is  produced 
which  ultimately  attains  a  relatively  considerable  size. 
It  remains,  however,  surrounded  by  the  leucoplast,  which 
gradually  becomes  much  stretched  until  there  is  merely 
a  thin  film  of  it  surrounding  the  striated  grain.  It  can 


236  VEGETABLE  PHYSIOLOGY 

frequently  only  be  detected  by  delicate  staining  as  the  starch 
grain  grows.  If  the  point  of  deposition  is  near  the  side 
of  the  leucoplast,  as  is  generally  the  case,  the  successive 
shells  of  starch  are  not  of  equal  width,  but  are  wider  on 
the  side  of  the  grain  which  is  in  relation  with  the  greater 
bulk  of  the  plastid.  The  amount  deposited  on  any  part 
of  the  first-formed  portion  is  proportional  to  the  thickness 
of  the  plastid  in  contact  with  that  part.  An  eccentric  shape, 
often  approximating  to  that  of  an  oyster-shell,  is  conse- 
quently arrived  at.  Even  the  most  eccentric  grains  can 
be  shown  by  delicate  staining  to  be  covered  entirely  by 
the  leucoplast,  even  the  small  free  end  which  appears  to 

protrude  from  the  latter  being 
clothed  by  a  thin  film  of  its 
substance. 

Some  grains  which  occur  in  the 
potato  are  not  so  simple  in  their 
structure.      Two  types  are  repre- 
FIG.  no.— A,  COMPOUND,  B,  SEMI-      sented  in  fig.  110,  A  and  B.     The 
COMPOUND    STARCH     GRAINS      f  ormer  arise  by  t wo  or  more  grains 

FROM  POTATO.  J 

originating  in  the  interior  of  a 

leucoplast :  as  each  grows  by  deposition  of  new  layers,  they 
become  closely  pressed  together,  and  constitute  a  compound 
grain.  Fig.  110,  B,  shows  what  is  often  called  a  semi-com- 
pound grain.  In  such  a  formation  an  ovoid  leucoplast  com- 
mences deposition  at  two  points,  one  towards  each  end.  As 
the  starch  is  deposited  round  each,  the  concentric  grains  come 
into  contact,  and  the  bulk  of  the  leucoplast  is  reduced  to 
a  shell  surrounding  the  mass.  Its  subsequent  continued 
activity  then  forms  new  sheaths  overlying  the  whole.  The 
leucoplast,  as  in  the  first  case,  is  gradually  used  up  by  its 
own  activity,  and  it  is  finally  reduced  to  a  film  of  extreme 
tenuity,  which  surrounds  the  whole  grain. 

A  very  curious  starch  grain  occurs  in  the  latex  of  certain 
species  of  Euphorbia,  having  the  appearance  of  a  dumb-bell 
(fig.  111).  This  also  is  formed  by  a  leucoplast ;  the  latter 
is  an  elongated  structure,  and  at  first  forms  a  rod  of  starch 


THE  STOKAGE  OF  KESEBVE  MATEEIALS    237 


along  its  axis.  As  the  deposition  proceeds  the  leucoplast 
becomes  very  much  stretched  longitudinally,  till  its  centre 
is  reduced  to  a  thin  film  round  the  rod  of  starch,  while 
what  is  left  of  its  substance  is  accumulated  at  the  two  ends. 
The  further  activity  of  these  portions  results  in  the  develop- 
ment of  the  two  heads  of  the  dumb-bell,  the  thin  film 
connecting  them  ceasing  to  deposit  any  starch  along  the 

centre  of  the  rod. 

It   is   not   very   easy   to 
see  the  leucoplasts  in  the 
potato ;    they   can   be   de- 
D  tected,  however,  more  easily 


Fio.     111.  —  LATICIFEROUS     CELL 
FROM     Euphorbia,     CONTAINING 

DUMB  -  BELL  -  SHAPED        STARCH 

GRAINS. 


FIG.  112.— GROUP  OF  ROD-LIKE 
LEUCOPLASTS,  I,  EACH  BEARING 
A  STARCH  GRAIN,  s,  COLLECTED 
ROUND  THE  NUCLEUS,  H,  OF  A 
CELL  OF  THE  PSEUDO-BULB  OF 
AN  ORCHID  (Phajus  grandifolius). 
X  500.  (After  Schimper.) 


in  other  plants.  Fig.  112  shows  a  group  of  them  forming 
starch  grains  in  a  cell  in  one  of  the  orchids.  The  greater 
bulk  of  each  lies  on  the  outside  of  the  grain  ;  they  are 
disc-like  in  shape  and  not  round  or  ovoid  as  in  the  potato. 

In  the  temporary  reservoirs  which  we  have  already 
noticed,  such  as  pollen  grains  and  tubes,  the  sheaths  of 
cells  in  various  regions  of  the  stem,  the  tissue  of  the  style  of 
the  lily,  &c.,  the  deposition  of  starch  is  not  caused  by  leuco- 
plasts but  by  the  general  protoplasm  of  the  cell.  In  these 
cases  immense  numbers  of  very  small  grains,  hardly  larger 
than  mere  specks,  make  their  appearance,  while  the  highest 
powers  of  the  microscope  fail  to't  enable  an  observer  to  detect 


288  VEGETABLE  PHYSIOLOGY 

the  presence  of  any  form  of  plastid  before  or  during  the 
deposition.  Instead,  the  minute  granules  can  be  seen 
to  arise  in  a  homogeneous  transparent  hyaline  protoplasm. 
The  same  phenomenon  occurs  in  connection  with  the 
deposition  of  starch  grains  in  the  cells  of  young  developing 
embryos,  in  the  early  stages  of  the  formation  of  the  seed, 
The  protoplasm  of  the  cells  may  be  seen  to  have  the  form 
of  a  coarse  network  with  many  small  meshes,  which  are 
empty  spaces  or  contain  only  cell-sap.  There  is  no  leuco- 
plast  inside  them,  nor  anything  comparable  to  one.  The 
starch  grains  originate  in  these  meshes  at  some  point  in 
contact  with  the  protoplasm  and  gradually  increase  in  size 
till  they  fill  them.  In  some  cases  simple,  in  others  com- 
pound, grains  of  starch  are  thus  developed. 

In  a  large  number  of  the  Fungi  which  store  up  carbo- 
hydrate reserve  materials,  these  take  the  form  of  glycogen. 
This  is  a  substance  which  presents  a  somewhat  close 
resemblance  to  starch,  being  readily  converted  into  sugar 
in  a  manner  almost,  if  not  quite,  identical  with  that  which 
is  characteristic  of  starch.  It  is  coloured  reddish-brown  by 
iodine.  It  is  usually  deposited  in  amorphous  form  in  the 
interior  of  the  fungal  hyphae,  or  of  particular  cells  of  them. 
In  a  few  cases  there  are  definite  granules,  which  to  a  certain 
extent  resemble  grains  of  starch,  and  which  have  been  stated 
to  originate  in  certain  corpuscular  bodies  resembling  leuco- 
plasts.  In  most  cases  the  deposition  appears  to  be  effected 
by  the  protoplasm. 

Another  carbohydrate  which  shows  a  certain  resem- 
blance to  starch,  though  perhaps  not  a  very  close  one,  is 
inulin.  The  distribution  of  this  material  is  much  more 
limited  than  that  of  starch,  but  it  is  known  to  occur  in 
several  groups  of  plants,  being  conspicuous  in  many  of  the 
Compositce  among  the  Dicotyledons,  and  in  several  species 
of  the  Liliacece,  Amaryllidacece,  and  other  allied  orders 
among  the  Monocotyledons.  Like  starch  and  glycogen,  it 
is  capable  of  transformation  into  a  sugar,  though  not  the 
same  sugar  as  in  the  other  cases.  It  exists,  in  the  plants 


THE  STOEAGE  OF  EESEEVE  MATEEIALS     239 

mentioned,  in  solution  in  the  cell-sap,  but  it  can  readily  be 
made  to  crystallise  out  or  to  be  precipitated  in  an  amor- 
phous condition  by  the  application  of  alcohol  (fig.  113). 

We  find  many  instances  of  the  occurrence  of  various 
sugars  as  reserve  materials.  Cane-sugar  is  present  in  large 
quantities  in  the  succulent  parenchyma  of  the  roots  of  the 
Beet  and  the  Mangel-wurzel,  and  of  the  stems  of  the  Sugar- 
cane ;  grape-sugar  is  found  in  the  leaves  of  the  bulbs  of 
the  Onion  and  allied  plants  ;  small  quantities  of  raffinose 
are  met  with  in  the  grains  of  barley  and  other  cereals. 


FIG.  113. — SPH.EBO-CKYSTALS  OF  INTJLIN  FROM  THE  ARTICHOKE. 

small  crystals  in  the  interior  of  cells  treated  with  alcohol;    B,  large  crystals 
extending  through  many  cells. 


These  are  all  present  in  solution  in  the  cell-sap,  as  has 
previously  been  mentioned. 

In  many  cases  carbohydrate  reserve  materials  are  found 
to  take  the  form  of  considerable  thickening  of  the  cell- 
walls.  That  these  are  really  deposited  in  seeds  with  a 
view  to  subsequent  utilisation  is  evident  from  a  study  of 
the  endosperm  of  many  palms,  the  cells  of  which  consist  of 
little  else  ;  the  walls  are  so  thick  that  the  cavities  are 
almost  obliterated,  and  the  small  space  that  is  left  between 
the  thickened  walls  contains  apparently  nothing  but  a 
small  amount  of  protoplasm  with  which  some  amorphous 
protein  matter  is  mixed.  Even  the  unthickened  cell-walls 
of  most  seeds  must  be  looked  upon  as  reserve  food  material, 


240 


VEGETABLE  PHYSIOLOGY 


as  they  are  used  up  in  nourishing  the  embryo  during  the 
early  stages  of  germination. 

It  is  necessary,  however,  to  mention  that  thickened 
cell-walls  must  not  always  he  regarded  as  stores  of  food. 
In  thickened  sclerenchymatous  tissue  and  in  ordinary  wood- 
cells  the  deposit  must  be  looked  upon  as  a  permanent 
strengthening  of  the  skeleton  of  the  plant. 

These  thickened  cell-walls  are  not  composed  always  of 
true  cellulose.  Our  knowledge  of  their  composition  is  not 


FIG.  114. — SECTION  THROUGH  EXTERNAL  REGION  OP  GRAIN 
OF  BARLEY. 

p,  pericarp  of  fruit ;  t,  testa  of  seed ;  al,  layer  of  cells 
containing  aleurone  grains ;  am,  cells  of  endosperm ; 
n,  nucleus.  (After  Strasburger.) 


at  all  complete,  but  it  extends  so  far  as  to  show  that  both 
cellulose  and  pectic  compounds  may  be  present  and  in  very 
different  proportions  in  different  cases.  Layers  of  muci- 
lage also  are  of  frequent  occurrence. 

Nitrogenous  material,  like  carbohydrate,  is  stored  up  in 
various  places  and  in  different  forms.  By  far  the  com- 
monest condition  is  that  of  some  description  of  protein. 
The  most  abundant  deposits  are  found  in  seeds,  in  the  cells 
of  which  they  usually  occur  in  the  form  of  granules  of 
varying  sizes  and  often  of  complex  composition.  In  certain 


THE  STOEAGE  OF  KESEKVE  MATEKIALS      241 


cases,  as  in  fleshy  roots,  the  protein  may  be  dispersed  in 
amorphous  form  in  the  substance  of  the  protoplasm. 

When  protein  is  stored  in  the  condition  of  granules 
these  are  known  as  aleurone  grains.  Like  starch  grains 
they  may  be  deposited  all  through  the  substance  of  the 
seed,  or  they  may  occupy  definite  layers,  as  they  do  in  the 
cereal  grasses  (fig.  114).  They  occur  sometimes  in  the  same 
cells  as  do  starch  grains,  as  in  the  pea  or  bean  (fig.  115). 
In  other  cases  they  are  found  associated  with  a  quantity 
of  oil,  as  in  the  seed  of  the  castor-oil  plant. 


FIG.    115. — CELLS  OP    EMBRYO    OF 

PEA.     (After  Sachs.) 
a,  aleurone  grains ;   st,  starch  grains. 


FIG.    116. — CELLS    OF  SEED 

OlfLupimiS,  SHOWING  COM- 
MENCING FORMATION  o» 
ALEURONE  GRAINS.  (After 
Rendle.) 

a,    nucleus ;    6,  vacuole ;    c, 
originating  aleurone  grain. 


An  instance  of  the  occurrence  of  aleurone  grains  of 
some  size,  but  yet  of  fairly  simple  composition,  is  afforded 
by  the  Lupin,  one  of  the  Leguminosce.  This  is  of  interest 
especially  because  the  origin  of  the  grain  can  be  observed 
and  its  development  traced.  In  this  seed  the  aleurone 
grains  begin  to  be  formed  at  a  very  early  period  of  the 
development,  just  as  the  growth  of  the  embryo  is  sufn 
ciently  advanced  to  swell  out  the  seed-coat.  The  cells  of 
the  embryo  at  that  period  show  the  protoplasm  not  sufficient 
in  amount  to  fill  each  cell,  so  that  a  number  of  spaces  or 
vacuoles  occur,  filled  with  sap.  At  certain  places  small 
projections  from  the  protoplasm  may  be  noticed  which  are 
of  spherical  or  ovoid  shape  (fig.  116,  c)  ;  these  gradually 

16 


242  VEGETABLE  PHYSIOLOGY 

increase  in  size,  growing  inwards  into  the  protoplasm  as 
well  as  outwards  into  the  vacuole,  till  they  can  be  seen  to 
be  in  the  form  of  grains  embedded  in  the  protoplasm,  which 
in  consequence  of  their  development  assumes  the  appear- 
ance of  a  coarse  network.  As  this  process  continues,  the 
original  grains  growing  in  size,  and  new  ones  being  con- 
stantly formed,  the  original  vacuoles  become  obliterated 
and  the  cell  swollen  out  by  its  own  deposits  (fig.  117). 
While  this  mechanical  process  is  going  on  chemical  changes 
also  take  place  in  the  material  secreted.  The  protoplasm 
forms  protein  originally  at  the  expense  of  the  amido-acids, 
sugars,  &c.,  brought  down  to  the  cell,  but  the  variety 


FIG.  117.— CELL  OF  RIPE  SEED  or       FIG.  118.— CELL  OF  RICINUS  SEED, 

LupinUS,  FILLED  WITH  ALEURONE  CONTAINING        FlVE        ALEUEONE 

GRAINS.  GRAINS. 


originally  constructed  is  not  necessarily  the  same  as  that 
subsequently  stored.  At  first  the  grains  are  not  soluble  in 
either  10  per  cent,  or  saturated  solutions  of  common  salt. 
Later  on  they  can  be  dissolved  by  both  of  these  fluids. 

The  deposition  of  aleurone  grains  in  the  cell  is  thus, 
like  that  of  starch,  a  process  of  secretion  carried  out  by  the 
protoplasm  :  a  process,  that  is,  of  manufacture  of  the  grain 
by  the  latter,  after  it  has  been  supplied  with  less  highly 
organised  material.  It  is  so  constructed  by  the  intervention 
of  the  protoplasm  itself,  the  grain  growing  at  the  apparent 
expense  of  the  substance  of  the  latter. 

There  is  no  doubt  that  the  amorphous  deposits  of  proteins 
in  the  cells  of  fleshy  roots  and  stems  are  due  to  a  similar 
process  of  secretion. 


THE  STOEAGE  OF  BESEBVE  MATEEIALS      243 

In  many  seeds,  among  which  may  be  mentioned  those 
of  the  Castor-oil  plant  and  the  Brazil  nut,  the  aleurone 
grains  possess  a  more  complicated  structure.  Fig.  118 
shows  a  section  of  one  of  the  cells  of  a  seed  of  the  castor-" 
oil  plant  in  which  some  of  them  are  lying.  The  figure 
represents  the  cell  after  treatment  with  alcohol,  and  sub- 
sequently with  water.  The  alcohol  removes  the  oil  with 
which  the  cells  are  filled,  and  which  obscures  the  appear- 
ance of  the  grains.  The  latter  are  of  ovoid  shape,  and  as 
they  lie  their  structure  is  not  apparent.  Water  dissolves 
part  of  the  outer  portion,  leaving  visible  the  ovoid  body, 
which  becomes  transparent.  Embedded  in  it  are  a  large 
regular  crystal  of  protein  matter,  and  a  small  rounded 
irregular  mass  consisting  mainly  of  minute  crystals  of  mineral 
matter.  These  two  constituents  are  spoken  of  as  the 
crystalloid  and  the  globoid  respectively.  The  part  of  the 
matrix  which  is  not  soluble  in  water  will  dissolve  in  a  10 
per  cent,  solution  of  common  salt,  while  the  crystalloid  is 
soluble  only  in  a  saturated  solution.  The  mineral  part  of 
the  globoid  of  the  grains  of  the  castor-oil  plant  is  a  double 
phosphate  of  magnesium  and  calcium. 

Examination  of  these  grains  and  their  reactions  shows 
that  several  proteins  can  be  detected  in  them.  Those 
soluble  in  water  are  proteoses,  while  the  others  which 
dissolve  only  in  salt  solutions  are  globulins.  In  grains 
met  with  in  other  plants,  metaproteins  occur  which  dissolve 
only  in  dilute  alkalies. 

Crystals  of  protein  occur  in  other  places  than  seeds. 
If  we  examine  a  young  potato,  we  find,  in  certain  cells 
lying  a  little  below  the  skin,  some  regular  transparent 
cubical  crystals,  which  are  composed  apparently  of  the 
same  material  as  the  crystalloids  of  the  complex  aleurone 
grains  described.  They  are  soluble  in  saturated  solutions 
of  common  salt.  Similar  crystals  are  met  with  in  the 
tissues  of  certain  seaweeds.  Many  of  them  can  be  made 
to  crystallise  from  the  solvents  which  are  used  to  extract 
them, 

16  * 


244  VEGETABLE  PHYSIOLOGY 

The  seeds  of  the  cereal  grasses  contain  two  other  very 
curious  reserve  proteins,  which  give  rise  in  the  flour  to  a 
peculiar  sticky  material  which  is  generally  known  as  gluten. 
They  do  not  appear  to  be  present  in  the  aleurone  grains 
of  the  seeds,  but  to  occur  in  the  starch- containing  cells. 
They  have  been  called  gliadin  and  glutenin ;  occurring 
separately  in  the  seed,  they  interact  with  one  another  in 
the  presence  of  water  and  form  the  gluten  of  the  flour. 
Like  the  zein  of  maize,  these  proteins  belong  to  the  peculiar 
class  whose  members  are  soluble  in  dilute  alcohol. 

In  many  cases  the  proteins  of  the  reservoirs  do  not  remain 
unchanged  during  the  resting  period  which  follows  their 
deposition.  This  is  especially  the  case  with  seeds,  in  which 
such  changes  are  characteristic  of  the  process  known  as 
ripening. 

Proteins  occur  also  in  the  temporary  reservoirs  to  which 
allusion  has  been  made.  Fleshy  roots  and  stems  contain 
them  in  amorphous  form  in  their  parenchyma  ;  certain 
forms  are  met  with  in  the  sieve- tubes,  and  are  coagulable  on 
boiling  like  the  globulins  of  the  seeds.  The  proteins  which 
are  constant  constituents  of  latex  are  no  doubt  in  great  part 
reserve  food-stuffs. 

In  many  cases  amido-acids  such  as  asparagin  may  be 
detected  in  the  sap  of  various  cells.  These  may  be  reserve 
materials  temporarily  retained  where  they  are  found,  or 
they  may  be  only  translocatory  products.  Their  occurrence 
in  some  resting  seeds  suggests  the  former  explanation  of 
their  presence.  It  is  not  easy  to  detect  them  in  the  cells, 
as  they  are  dissolved  in  the  sap,  but  in  many  cases  they  can 
be  caused  to  crystallise  by  placing  a  section  of  the  tissue 
on  a  glass  slip  in  glycerine. 

A  great  many  plants  store  quantities  of  complex  sub- 
stances known  as  glucosides.  These  are  bodies  which  on 
decomposition  give  rise  to  some  kind  of  sugar,  together  with 
other  products  usually  belonging  to  the  aromatic  series 
of  carbon  compounds.  Among  them  may  be  mentioned 
amygdalin,  which  is  found  in  the  seeds  of  the  bitter  almond. 


THE  STOEAGE  OF  EESEEVE  MATEEIALS      245 

During  germination  it  splits  up  into  benzoyl  aldehyde, 
hydrocyanic  (prussic)  acid,  and  grape-sugar.  Many  such 
bodies  are  known,  and  they  are  somewhat  widely  distri- 
buted. Some  occur  in  seeds,  but  they  are  more  frequently 
represented  in  the  reservoirs  contained  in  fleshy  roots  and 
stems.  Many  plants  belonging  to  the  Cruciferce  and  several 
allied  orders  are  particularly  rich  in  reserve  materials  belong- 
ing to  this  group.  Sinigrin,  or  myronate  of  potash,  is  the 
principal  glucoside  which  they  contain.  It  splits  up  into 
sulphocyanate  of  allyl,  grape-sugar,  and  hydrogen-potassium- 
sulphate. 

The  nutritive  value  of  these  bodies  is  partly  due  to  the 
sugar  which  they  yield  on  decomposition.  The  evidence 
that  the  other  products  can  minister  to  nutrition  is  not 
very  complete,  though  it  seems  satisfactory  in  certain 
cases. 

Fats  or  oils  are  frequently  stored  as  reserve  food-stuffs 
in  different  plants.  The  distribution  of  this  material  is 
very  varied,  though,  as  in  so  many  other  cases,  the  seed  is 
the  most  general  place  of  deposition.  Many  seeds — that 
for  instance  of  the  castor-oil  plant — contain  as  much  as 
60  per  cent,  of  their  dry  weight  of  oil,  which  is  non-volatile. 
Others  contain  as  little  as  2  per  cent.,  and  between 
these  limits  very  varying  amounts  may  be  found.  When 
the  oil  is  in  great  preponderance,  it  is  usual  for  no  other 
form  of  carbonaceous  reserve  to  be  present ;  in  cases  where 
but  little  oil  occurs  starch  is  usually  found  as  well,  as  in  so 
many  of  the  Leguminosce.  The  Cruciferce  as  a  group  often 
contain  oil  in  fairly  large  quantity.  As  a  rule  nitrogenous 
reserves  in  the  shape  of  aleurone  grains  accompany  the  oil. 

In  other  places  than  seeds  large  deposits  of  oil  often 
occur,  though  their  purpose  is  not  so  obvious.  We  have 
them  in  large  amount  in  the  pericarps  of  certain  fruits, 
such  as  the  olive  ;  in  the  petals  of  many  flowers,  e.g.  Funkia 
and  Ornithogalum  ;  in  the  leaves  of  some  of  the  Agaves,  the 
roots  of  Oncidium,  &c.  They  can  hardly  be  regarded  in 
some  cases  as  truly  reserve  materials,  being  perhaps  more 


246  VEGETABLE  PHYSIOLOGY 

strictly  connected  with  the  mechanisms  of  dispersion  of 
seeds. 

The  mode  of  deposition  of  oil  or  fat  is  not  at  all  well 
known.  It  is  generally  found  saturating  the  protoplasm  of 
the  cell  in  which  it  lies,  and  not  occupying  a  definite  space 
as  do  aleurone  and  starch  grains.  Whether  it  is  secreted 
from  the  substance  of  the  protoplasm,  or  whether  the 
materials  of  which  it  is  made  are  taken  to  the  latter  in  a 
state  near  the  condition  of  the  finished  fat,  is  uncertain.  It 
is  formed  by  the  combination  of  a  fatty  acid  with  gly- 
cerine. Both  these  bodies  can  be  formed  in  the  plant,  but 
how  they  are  finally  presented  to  us  in  the  shape  of  oil  is 
still  in  need  of  elucidation.  As  the  oil  appears  in  the  cell 
it  seems  to  point  to  a  process  of  breaking  down  of  the 
protoplasm  itself,  and  not  to  a  direct  combination  of  the 
antecedents  mentioned.  If  we  stain  cells  which  are  forming 
fat  with  osmic  acid,  which  colours  fatty  bodies  brown  or 
black,  we  see  in  the  protoplasm  small  specks  of  fatty 
matter,  which,  while  in  the  youngest  cells  mere  dots,  are 
in  older  ones  larger,  and  can  be  recognised  as  droplets.  In 
still  older  ones  the  blackness  permeates  the  whole  proto- 
plasm, indicating  that  the  latter  is  saturated  with  the  oil, 
the  droplets  having  run  together  in  consequence  of  their 
number  and  dimensions. 

The  appearances  are,  however,  not  inconsistent  with  the 
view  that  the  work  of  the  protoplasm  is  only  to  effect 
the  ultimate  changes,  or  interaction  of  the  glycerine  and 
the  fatty  acids,  which  are  transported  separately  to  the  cells 
or  perhaps  formed  there  from  some  antecedent. 

The  deposition  of  fat  in  some  cases,  particularly  in 
leaves,  has  been  stated  to  be  effected  by  the  agency  of  certain 
plastids  corresponding  to  the  leucoplasts  already  mentioned 
in  connection  with  the  formation  of  starch  grains.  These 
structures,  which  have  been  called  elaioplasts,  are  curious 
bodies  of  various  shapes,  sometimes  round  or  oval,  some- 
times irregular  in  contour,  which  lie  near  the  nucleus  of 
the  cell.  Like  the  other  plastids,  they  consist  of  a  spongy 


THE  STOEAGE  OF  EESEEVE  MATEEIALS     247 

protoplasmic  framework,  in  the  meshes  of  which  the  oil  is 
formed,  much  as  it  is  in  the  protoplasm  of  the  seeds  already 
described. 

All  these  bodies,  when  acted  upon  by  a  process  ana-  - 
logous  to  the  digestion  known  in  the  animal  kingdom,  are 
converted  into  materials  which  can  directly  nourish  the 
living  substance,  or  can  be  transported  easily  about  the 
plant  in  a  condition  of  which  the  latter  can  readily  take 
advantage,  needing  indeed  very  little  constructive  change 
to  fit  them  for  actual  assimilation. 


248  VEGETABLE  PHYSIOLOGY 


CHAPTEE  XVI 

DIGESTION 

We  have  noticed  in  studying  the  deposition  of  reserve 
food-stuffs  that  the  forms  in  which  they  exist  in  the  reser- 
voirs differ  in  many  respects  from  those  which  they  assume  for 
purposes  of  transport  or  translocation.  They  are  generally 
insoluble  in  water  or  cell-sap,  and  almost  always  indiffu- 
sible,  whereas  they  travel  in  the  form  of  soluble,  diffusible, 
substances.  The  removal  of  them  from  the  seats  of  storage 
takes  place  at  times  which  are  dependent  on  the  resump- 
tion of  activity  of  growth  or  development ;  and  as  such  a 
removal  involves  the  resumption  of  the  travelling  forms, 
they  must  undergo  a  process  which,  from  analogy  with 
similar  processes  in  the  animal  body,  may  be  described  as 
digestion.  Each  must,  after  such  treatment,  be  presented 
to  the  protoplasm  of  the  growing  cells  in  much  the  same 
form  or  condition  as  that  in  which  it  was  first  constructed 
from  the  simple  bodies  which  the  plant  absorbed  from  its 
environment.  This  is  necessary  in  all  cases,  because, 
as  we  have  already  noticed,  the  storage  forms  are  not 
directly  assimilable  by  the  protoplasm,  but  have  undergone 
a  certain  modification  in  the  process  of  their  deposition. 

The  process  of  digestion  in  plants  is  chiefly  intra- 
cellular,  and  takes  place  in  all  cells  in  which  reserve 
materials  occur.  It  is  only  occasionally  that  we  find  it 
taking  place  on  the  exterior  of  the  plant — that  is,  not  in 
the  interior  of  a  cell.  In  a  few  cases  we  find  it  carried 
on  in  connection  with  the  absorption  of  nitrogenous  or 


DIGESTION  249 

protein  food,  as  has  been  already  shown  in  a  preceding 
chapter.  Digestion,  though  most  generally  associated  in 
plants  with  the  utilisation  of  reserve  materials,  may  thus 
occasionally  he  met  with  in  connection  with  the  absorption 
of  food  from  without,  when  it  is  a  process  precisely  similar 
to  the  digestive  processes  of  the  higher  animals,  though 
it  is  somewhat  simpler  in  the  details  of  its  mechanism. 

The  intra-cellular  digestion  of  plants  agrees  very  closely 
with  that  of  many  of  the  humbler  animals,  and  corresponds 
also  with  such  processes  in  the  higher  forms  as  the  utilisa: 
tion  of  the  glycogen  of  the  liver  and  the  fat  of  various 
regions. 

We  have  seen  that  in  a  few  rare  cases  protein  material 
is  absorbed  into  the  plant-body  through  various  leaves 
or  modified  foliar  organs.  The  insectivorous  plants  are 
materially  assisted  in  their  growth  by  capturing  and  digest- 
ing various  insects,  the  products  of  the  digestion  being 
absorbed  by  the  surface  of  the  leaf  or  other  organ  concerned. 
We  examined  several  of  these  mechanisms  in  some  detail 
in  Chapter  XIV. 

Absorption  of  food  from  without,  after  preliminary  diges- 
tion, is  much  more  frequently  observed  when  we  study  the 
nutritive  processes  of  the  Fungi.  Not  only  protein,  but 
also  carbohydrate  and  fatty  substances  are  thus  digested 
outside  the  body  of  the  plant,  and  the  products  of  the  diges- 
tion are  subsequently  absorbed. 

We  have  then  to  inquire  how  these  processes  of  diges- 
tion, whether  internal  or  external,  are  brought  about. 

The  protoplasm  of  the  cell,  among  its  many  properties, 
no  doubt  has  the  power  of  setting  up  these  decompositions, 
and  probably  in  many  of  the  very  lowly  plants,  in  which 
the  whole  organism  consists  of  only  a  few  protoplasts  or 
perhaps  a  single  one,  the  work  is  altogether  effected  by 
its  instrumentality.  The  protoplast,  in  fact,  carries  out 
all  the  various  processes  of  life  by  the  interactions  of  its 
own  living  substance  with  the  materials  absorbed  by  it, 
aided  in  the  constructive  processes  by  the  chlorophyll 


250  VEGETABLE  PHYSIOLOGY 

apparatus,  if  it  possesses  one.  In  such  a  protoplast  we 
may  observe  at  times  the  storage  of  such  a  reserve 
material  as  starch,  and  its  digestion  at  the  appropriate 
period. 

Even  in  more  complex  plants  it  is  certain  that  the  living 
substance  of  every  protoplast  is  in  a  constant  state  of 
change,  initiating  many  decompositions  in  which  its  own 
substance  takes  part  as  well  as  others,  into  the  course  of 
which  it  does  not  itself  enter.  Among  these  decompositions 
we  must  include  the  various  intra-cellular  digestive  processes. 

Though  all  protoplasm  has  this  power,  it  is  not  usual 
in  plants,  any  more  than  in  animals,  to  find  it  exclusively 
relying  on  it.  The  work  of  digestion,  at  any  rate,  is  generally 
carried  out  by  peculiar  substances  which  it  forms  or  secretes 
for  the  purpose.  We  have  in  plants  a  large  number  of 
these  secretions,  which  are  known  as  enzymes  or  soluble 
ferments. 

The  action  of  these  enzymes  is  not  at  all  completely 
understood.  They  appear  not  to  enter  into  the  composi- 
tion of  the  substances  which  are  formed  by  their  activity, 
and  they  seem  to  be  capable  of  carrying  out  an  almost  in- 
definite amount  of  such  work  without  being  used  up  in  the 
process.  They  are  inactive  at  very  low  temperatures,  but 
effect  the  decompositions  they  set  up  freely  at  the  ordinary 
temperature  of  the  plant.  As  the  temperature  at  which 
they  are  working  is  raised,  their  activity  increases  up  to  a 
certain  point,  which  varies  slightly  for  each  enzyme,  and 
is  called  its  optimum  point.  This  usually  ranges  between 
80°  and  45°  C.  If  the  temperature  is  raised  above  the 
optimum  point,  the  enzyme  becomes  less  and  less  active  as 
it  rises,  and  at  about  60°-70°  C.  it  is  destroyed.  The  exact 
point,  however,  varies  a  good  deal  in  the  cases  of  different 
enzymes. 

Enzymes  work  most  advantageously  in  darkness  or  in 
a  very  subdued  light ;  if  they  are  exposed  to  bright  sun- 
shine they  are  gradually  decomposed,  the  violet  and  ultra- 
violet rays  being  apparently  most  powerful  in  effecting 


DIGESTION  251 

their  destruction.  They  are  often  injuriously  affected  by 
neutral  salts,  alkalies,  or  acids,  though  in  this  respect  there 
exists  considerable  diversity  throughout  the  group. 

The  enzymes  are  manufactured  by  the  protoplasm  of 
the  various  cells  in  which  they  occur,  being  produced  from 
its  own  substance,  in  a  manner  somewhat  similar  to  that 
of  the  formation  of  the  cell- wall.  Usually  their  presence 
is  accompanied  by  a  marked  granularity  of  the  protoplasm, 
due  to  the  formation  in  it  of  an  antecedent  substance,  known 
as  a  zymogen,  which  is  readily  converted  into  the  enzyme. 
This  granularity  does  not,  however,  always  occur,  though 
we  have  reason  to  suppose  that  the  secretion  of  the  enzyme 
always  takes  place  by  successive  stages.  The  zymogen 
has  not,  however,  been  definitely  detected  in  all  cases. 

We  find  various  degrees  of  completeness  of  differentia- 
tion of  the  cells  which  produce  these  enzymes.  In  the 
simplest  cases,  such  as  the  mesophyll  of  the  leaves  of  most 
plants,  or  the  great  majority  of  seeds,  or  the  tubers  of  the 
potato,  the  enzyme  is  found  in  all  the  cells  which  contain 
the  reserve  materials,  so  that  a  rapid  transformation  of 
the  latter  is  readily  possible.  In  the  Horse-radish  and 
many  allied  plants  the  cells  which  secrete  the  enzyme  do 
not  themselves  contain  any  reserve  materials,  but  are 
situated  among  those  which  do,  so  that  the  enzyme  has  to 
pass  from  the  seat  of  its  formation  to  other  cells  in  order 
to  discharge  its  function.  This  is  a  very  slow  and  gradual 
process,  and  is  probably  carried  out  through  the  agency  of 
the  delicate  filaments  of  protoplasm  which  extend  through 
the  cell-walls,  for  enzymes  are  not  capable  of  dialysing 
through  a  membrane. 

The  occurrence  of  such  cells,  which  are  apparently  set 
apart  especially  for  the  secretion  of  an  enzyme,  gives  us, 
as  it  were,  the  starting  points  of  the  special  structures 
known  as  glands,  whose  function  is  similar,  but  whose 
structure  is  more  complex.  In  some  of  the  plants  belong- 
ing to  the  natural  orders  Capparidacece  and  Tropceolacece. 
the  glandular  cell  divides  several  times  to  form  a  little  mass 


252 


VEGETABLE  PHYSIOLOGY 


or  nodule  of  secreting  cells,  which  must  be  regarded  as  a 
rudimentary  gland,  though  it  is  not  provided  with  any 
definite  outlet  or  duct. 

In  the  seed  of  the  cereal  grasses  there  is  a  special  organ 
separating  the  embryo  from  the  endosperm.  This  structure, 
which  is  a  modification  of  part  of  the  cotyledon,  is  known 
as  the  scutellum  (fig.  119 ) ;  its  function  is  to  effect  the 
absorption  of  the  nutritive  material  of  the  endosperm,  and 
supply  it  to  the  growing  embryo.  This  scutellum  is  covered 
on  its  outer  face,  which  is  in  contact  with  the  endosperm, 
by  a  layer  of  cylindrical  cells,  whose  long  axis  is  at  right 


FIG.  119. — SECTION  OF  OAT-GRAIN. 
9,  plumule;   r,  radicle;   *,    scutellum. 


FIG.  120. — SECTION  OF  PORTION  OF 
SCUTELLUM  OF  BARLEY,  SHOWING 
THE  SECRETING  EPITHELIUM. 


angles  to  the  surface  (fig.  120).  These  cells  are  very 
granular  in  appearance,  and  form  a  very  marked  secreting 
tissue,  producing  two  enzymes,  which  are  subsequently 
discharged  into  the  endosperm  to  effect  the  digestion  which 
must  precede  absorption.  The  aleurone  layer  of  the  same 
grain  (fig.  121),  which  has  already  been  described,  is  also 
a  secreting  layer,  resembling  the  outer  layer  of  the  scutellum 
in  several  respects. 

The  tentacles  of  the  leaves  of  Drosera,  to  which  allusion 
has  already  been  made,  are  very  definitely  secreting  struc- 
tures ;  in  addition  to  preparing  an  enzyme  they  produce 
a  weak  acid,  both  of  which  are  present  in  the  glairy  material 
that  they  pour  out  over  the  captured  insect.  These  tentacles 


DIGESTION 


258 


(fig.  122)  and  the  secreting  structures  of  the  leaves  of  Dioncea 
and  other  plants,  as  well  as  the  similar  bodies  which  occur 


Fia.  121. — SECTION  THROUGH  EXTERNAL  REGION  OF  GRAIN 
OF  BARLEY. 

p,  pericarp  of  fruit ;  t,  testa  of  seed ;  al,  layer  of  cells 
containing  aleurone  grains ;  am,  cells  of  endosperm ; 
TO,  nucleus.  (After  Strasburger.) 

in  the  lining  of  the  pitcher  of  Nepenthes,  must  be  regarded 
as  actual  glands,  comparable  to  those 
of  the  alimentary  canal  of  the  animal 
body,  though  less  complex  in  structure. 
Glandular  hairs,  which  consist  of  a  few 
cells  situated  on  a  stalk,  are  found  in 
great  numbers  on  other  plants,  especi- 
ally some  species  of  Saxifraga. 

There  are  many  of  these  enzymes 
present  in  different  plants,  the  function 
of  some  of  which  is  still  not  understood. 
Many,  however,  have  been  investigated 
with  some  completeness.  They  are 
usually  classified  according  to  the  mate- 
rials on  which  they  work.  We  may 
describe  here  four  groups,  the  members 
of  which  take  part  in  the  digestion  of  reserve  materials, 
as  well  as  in  the  processes  of  external  digestion.  These 


FIG.  122.— GLANDULAR 
APEX  OF  A  TENTACLE 
OF  Drosera. 


254  VEGETABLE  PHYSIOLOGY 

decompose  respectively  carbohydrates,  proteins,  glucosides, 
and  fats  or  oils.  In  nearly  every  case  the  action  of  these 
enzymes  is  one  of  hydration,  the  body  acted  upon  being 
made  to  take  up  water,  and  to  undergo  a  subsequent 
decomposition. 

Of  those  which  act  upon  carbohydrates  we  have  two 
varieties  of  diastase,  which  convert  starch  into  maltose,  or 
malt-sugar  ;  inulase,  which  forms  another  sugar,  levulose 
or  fructose,  from  inulin  ;  invertase,  which  converts  cane- 
sugar  into  glucose  and  fructose  ;  glucase  or  maltase,  which 
produces  grape-sugar  from  maltose ;  and  cytase,  which 
hydrolyses  cellulose.  Another  enzyme,  which  does  not 
appear  to  be  concerned  with  digestion  so  directly  as  the 
others,  is  known  as  pectase  ;  it  forms  vegetable  jelly  from 
pectic  substances  occurring  in  the  cell-wall. 

The  members  of  the  second  group  act  upon  protein 
substances,  and  are  technically  known  as  proteoclastic 
enzymes.  The  principal  members  of  this  group  are  peptase, 
the  various  tryptases,  and  ereptase.  Peptase  and  tryptase 
convert  albumins  and  globulins  into  peptones,  the  tryptases 
also  decomposing  certain  peptones  into  amino-  and  amido- 
acids ;  while  ereptase  has  only  the  power  of  effecting  the 
last-named  change.  Allied  to  these  is  rennet,  which  converts 
the  caseinogen  of  milk  into  casein,  the  characteristic  protein 
of  cheese.  It  occurs  in  a  great  many  plants,  but  its  function 
in  vegetable  metabolism  is  unknown. 

The  enzymes  which  act  upon  glucosides  are  many  ;  the 
best  known  are  emulsin  and  myrosin  ;  other  of  less  frequent 
occurrence  are  eryihrozym,  rhamnase,  and  gaultherase. 
Those  which  decompose  fats  have  not  been  so  fully  inves- 
tigated :  they  are  known  as  Upases,  but  whether  there  are 
many  different  varieties  or  not  has  not  at  present  been 
ascertained. 

Diastase  appears  to  exist  in  two  varieties,  distinguished 
from  each  other  by  their  mode  of  action  on  the  starch 
grain.  One,  called  diastase  of  translocation,  dissolves  the 
grain  slowly  from  without  inwards,  without  altering  its 


DIGESTION  255 

general  appearance  ;    the  other,  diastase   of  secretion,  dis- 
integrates it  by  a  process  of  corrosion  before  dissolving  it 
(fig.  123).     The  first  of  the  varieties  has  a  very  wide  distribu- 
tion in  plants,  being  present  almost 
everywhere.     The  second  is  the  body         fi\     /s\ 
formed    by    the    glandular    covering          (=/      \S 
or    epithelium    of    the    scutellum    of  &      ^ 

the  grasses.  §)       ^ 

The  great  function  of  diastase  in      FlG-  123— STARCH  GRAINS 

.  IN    PROCESS    OF  DIGES- 

the  plant  is  to  transform  starch  (and         HON.   THE  SUCCESSIVE 
probably   glycogen  where   it   occurs)        FIGURES  SHOW  THE  PRO- 

J     °  J  GRESS  OF  DISSOLUTION. 

into  maltose  or  malt-sugar.  Wher- 
ever starch  is  formed,  whether  in  the  living  leaf  or  in  the 
reservoir  set  apart  for  storage,  it  must  be  regarded  as  a 
reserve  material,  and  its  removal  from  the  seats  of  deposi- 
tion is  preceded  by  its  conversion  into  this  sugar.  The 
details  of  the  transformation  are  not  fully  known  at  present, 
but  a  good  deal  of  information  has  been  obtained  through 
the  labours  of  many  observers.  Starch  has  a  rather  large 
molecule,  but  its  exact  formula  is  not  thoroughly  known. 
For  a  long  time  it  was  taken  to  be  approximately  w(C6H1005), 
and  the  value  of  n  was  thought  to  be  5.  More  recently  the 
suggestion  has  been  made  that  the  molecule  is  much  larger, 
and  may  be  more  truly  represented  by  5[(Ci2H20010)2o]j  the 
view  being  based  upon  the  formation  of  several  complex 
substances  during  its  decomposition.  The  starch  molecule 
is  possibly  composed  of  four  dextrin-like  groups,  each 
(C12H2o010)oo  arranged  about  a  fifth.  It  has  been  suggested 
that  the  first  action  of  the  diastase  is  the  liberation  of  these 
from  one  another  ;  and  that  four  of  them  by  successive 
incorporations  of  water  are  converted,  through  a  series  of 
complex  substances  called  malto-dextrins,  into  maltose, 
while  the  fifth  withstands  the  action  of  the  enzyme  for  a 
considerable  time.  After  the  action  of  the  diastase  has 
been  proceeding  for  some  time  the  resulting  product  is 
found  to  be  four  parts  maltose  and  one  part  dextrin. 

How  far  this  series  of  decompositions  represents  what 


256  VEGETABLE  PHYSIOLOGY 

takes  place  in  the  plant  is  uncertain,  but  it  is  clear  that  the 
starch,  which  is  insoluble,  is  converted  into  sugar,  which 
can  be  removed  to  the  parts  of  the  plant  where  it  is  required 
for  building  up  the  protoplasm. 

Inulase  occurs  in  the  tubers  and  tuberous  roots  of  some 
of  the  Composite,  in  the  bulbs  of  certain  Monocotyledons, 
and  in  some  of  the  Fungi.  It  converts  inulin  ultimately 
into  levulose  or  fructose,  but  the  action  is  not  a  very  simple 
one,  at  least  one  intermediate  body  being  formed  during 
the  process. 

Invertase  has  a  much  wider  distribution.  It  is  easily 
extracted  from  the  Yeast-plant,  in  which  it  is  present  in 
relatively  considerable  quantity.  Other  fungi  which  con- 
tain it  are  Fusarium  and  Aspergillus,  besides  certain  bacteria. 
In  flowering  plants  it  has  been  found  in  seeds,  buds,  leaves, 
stems,  roots,  and  pollen  grains.  Its  action  is  the  hydrolysis 
of  cane-sugar,  which  it  splits  up  into  glucose  and  fructose, 
according  to  the  equation 


C6H1206  +    C6H18Ofl 

Cane-sugar  Water  Glucose  Fructose 

Glucase  occurs  in  the  grains  of  various  cereals,  being 
especially  prominent  in  the  Maize.  It  is  also  fairly  abundant 
in  the  Yeast-plant.  It  has  no  action  on  cane-sugar,  but 
splits  up  maltose  into  glucose,  one  molecule  of  the  former 
taking  up  water  and  yielding  at  once  two  molecules  of 
the  latter. 

Other  sugars  of  similar  constitution  to  maltose  and 
cane-sugar  are  made  to  undergo  similar  transformations  by 
enzymes  of  less  widespread  distribution.  The  chief  of  these 
are  trehdlase,  raffinase  or  melibiase,  melizitase,  and  lactase. 

There  appear  to  be  several  varieties  of  cytase,  which 
can  be  prepared  from  various  seeds.  The  enzyme  was 
first  discovered  in  the  germinating  grain  of  the  barley,  in 
which  it  is  located  chiefly  in  the  aleurone  layer  and  to  a 
less  extent  in  the  epithelium  of  the  scutellum,  where  it 
exists  side  by  side  with  diastase.  It  dissolves  the  walls  of 


DIGESTION  257 

the  cells  of  the  endosperm,  detaching  them  from  each  other 
and  giving  a  curious  mealy  character  to  the  grain.  Its 
presence  was  first  suspected  in  the  Date-palm,  where  large 
reserves  of  cellulose  are  found  in  the  hard  cell-walls  of  the 
endosperm.  The  embryo  dissolves  these  walls  and  absorbs 
their  products,  the  work  being  effected  by  an  epithelium 
which  covers  the  part  of  the  cotyledon  which  remains  in 
the  seed  during  the  early  processes  of  germination.  This 
epithelium  is  composed  of  elongated  cells  arranged  in  a 
manner  resembling  that  characteristic  of  those  which  form 
the  secreting  layer  of  the  scutellum.  It  has  recently  been 
shown  that  cytase  is  formed  in  the  embryo,  probably  in  this 
layer,  and  passes  thence  into  the  endosperm.  The  amount 
of  it  that  can  be  detected  is  very  small,  however,  and  the 
process  of  the '  decomposition  of  the  cellulose  is  very  slow 
and  gradual.  Cytase  exists  in  considerable  quantity  in 
some  of  the  higher  fungi  and  in  certain  bacteria. 

Pectase  has  recently  been  found  to  be  very  widespread 
in  plants.  Its  function  is  not  very  clear,  but  it  may  assist 
cytase  in  the  swelling  up  of  the  cell-wall  which  is  antecedent 
to  solution.  It  is  recognised  by  its  power  of  forming  vege- 
table jelly  from  the  pectic  substances  of  the  cell- wall.  This 
jelly  appears  to  be  a  compound  of  pectic  acid  and  calcium. 

The  enzymes  which  digest  proteins  are  frequently  on 
that  account  spoken  of  as  proteoclastic  enzymes.  There  are 
three  main  classes  of  them  known  at  present.  The  first,  the 
peptases,  represented  by  the  pepsin  of  the  stomach  of  the 
higher  animals,  converts  albumins,  globulins,  and  certain 
insoluble  proteins  into  peptones,  several  intermediate  bodies, 
known  as  proteoses  or  albumoses,  being  formed  during  the 
process.  The  members  of  the  second  group,  the  tryptases, 
which  may  be  represented  by  the  trypsin  of  the  pancreas, 
carry  the  digestion  further  and  split  up  certain  peptones  into 
amino-  and  amido-acids,  of  which  the  chief  that  have  been 
observed  are  leucin,  tyrosin,  and  asparagin.  Those  of  the 
third  class,  the  ereptases,  decompose  peptone  with  the 
formation  of  the  same  amino-  and  amido-acids. 

17 


258  VEGETABLE  PHYSIOLOGY 

It  is  not  quite  certain  that  representatives  of  the  first 
class  are  to  be  met  with  in  plants.  It  is  for  the  present 
probable,  however,  that  the  enzyme  of  some  insectivorous 
plants  is  a  peptase.  It  acts  only  in  the  presence  of  a  weak 
acid,  as  does  the  pepsin  of  the  stomach,  but  the  products 
which  it  forms  have  not  been  accurately  investigated.  It 
is  apparently  only  secreted  when  the  gland  has  been  stimu- 
lated by  the  absorption  of  nitrogenous  matter. 

Several  varieties  of  vegetable  tryptase  have  been  dis- 
covered and  their  properties  investigated.  The  earliest 
known  enzyme  belonging  to  the  group  is  the  papam  which 
has  been  extracted  from  the  Papau  (Carica  Papaya).  It 
appears  to  exist  in  greatest  quantity  in  the  pulp  of  the 
fruit,  but  is  present  also  in  the  sap  which  can  be  expressed 
from  the  stem  and  leaves.  It  is  apparently  associated  in 
the  juice  with  a  peculiar  proteose  or  albumose,  and  it  is 
most  energetic  in  a  neutral  solution,  though  it  can  act 
also  in  a  faintly  alkaline  one.  It  is  easily  destroyed  by  a 
very  small  trace  of  free  acid. 

Another  tryptase,  which  has  been  named  bromelin,  has 
been  extracted  from  the  fleshy  pulp  of  the  Pine-apple 
(Ananassa  sativa).  Like  papain  it  is  associated  with  a 
proteose.  It  acts  most  energetically  in  neutral  and  faintly 
acid  solutions,  alkalies  in  very  small  traces  being  preju- 
dicial to  it.  Its  activity  varies  a  good  deal  according  to 
the  acid  which  is  present,  and  to  some  extent  according  to  the 
protein  which  it  is  digesting. 

Other  vegetable  tryptases  have  been  extracted  from  the 
germinating  seeds  of  the  Lupin,  the  seedlings  of  several 
plants,  the  fruit  of  the  Kachree  gourd  (Cucumis  utilissimus), 
the  juice  of  the  Fig-tree  (Ficus  carica),  and  the  leaves  of 
certain  species  of  Agave.  How  far  these  are  identical,  or 
whether  they  present  specific  differences,  appears  at  present 
uncertain.  They  are  all  active  in  faintly  acid  solutions, 
but  the  most  favourable  concentration  appears  to  vary. 
The  enzyme  of  the  Kachree  gourd  is  most  effective  when 
the  medium  is  faintly  alkaline,  whereas  that  of  the  lupin 


DIGESTION  259 

seed  is  inoperative  under  these  conditions.  Too  much 
stress  must  not,  however,  be  laid  upon  this  point,  as  the 
enzymes  have  not  been  prepared  in  any  case  in  anything 
like  a  pure  condition. 

Eecently  Vines  has  found  that  members  of  the  ereptase 
class  are  very  widespread  in  plants,  occurring  in  almost  all 
parts  of  them.  His  researches  suggest  that  possibly  the 
so-called  tryptases  are  mixtures  of  peptase  and  ereptase. 

The  action  of  all  these  proteoclastic  enzymes  is  probably 
one  of  hydrolysis,  though  it  is  difficult  to  prove  it  by  analysis. 

Kennet  occurs  in  many  seeds,  in  some  cases  in  the  germinat- 
ing, and  in  others  in  the  resting,  condition.  It  has  also  a 
wide  distribution  in  the  vegetative  and  floral  parts  of  various 
plants.  Whether  it  is  really  proteoclastic  in  the  vegetable 
organism  is  hard  to  say,  as  the  details  of  its  action  are 
unknown.  It  is  so  in  the  animal  body. 

The  enzymes  which  decompose  glucosides,  as  we  have 
already  seen,  are  numerous  and  varied  in  their  distribution, 
occurring  in  various  fungi  and  lichens  as  well  as  in  the  higher 
plants.  Their  action  may  be  illustrated  by  the  behaviour 
of  emulsin,  which  exists  in  quantity  in  the  seeds  of  the  bitter 
Almond  and  in  the  vegetative  parts  of  the  Cherry-laurel 
(Prunus  Laurocerasus)  .  It  splits  up  the  glucoside  amygdalin 
according  to  the  equation 

C20H27NOn  +  2H30  =  C7H40  +  HCN  +  2(C6H1206) 

Amygdalin  Benzole  Prussic  Glucose 

aldehyde  acid 

This  is,  as  in  other  cases,  a  process  of  hydrolysis.  Myrosin, 
another  of  the  group,  is  peculiar  in  that  it  effects  its  character- 
istic decomposition  without  causing  the  incorporation  of 
water  during  the  process,  thus  : 


o  =  C3H5CNS  +  C6H1206  +  KHS04 

Sinigrin  Sulpho-cyanate  Glucose  Potassium- 

of  allyl  hydrogen- 

sulphate 

Other,  such  as  rhamnase,  existing  in  the  seeds  of  Ehamnus 
infectorius,   erythrozym  in  the  Madder,   gaultherase  in   the 

17  * 


260  VEGETABLE  PHYSIOLOGY 

bark  of  Betula  lenta,  act  on  various  glucosides,  after  the 
manner  of  emulsin. 

The  digestion  of  the  glucosides,  we  may  notice,  is  always 
accompanied  by  the  appearance  of  sugar,  which  is  one 
of  the  products  of  their  decomposition.  The  fate  of  the 
other  bodies  into  which  they  split  is  not  well  ascertained, 
though  there  is  some  evidence  that  cyanogen  compounds, 
even  such  as  hydrocyanic  or  prussic  acid,  are  used  for 
nutritive  purposes  by  certain  plants. 

The  digestion  of  fat  or  oil  has  not  been  very  fully  in- 
vestigated, though  certain  facts  are  known  concerning 
its  fate  in  germinating  seeds.  The  digestion  is  generally 
accompanied  by  the  appearance  of  starch  grains  in  cells 
near  the  seat  of  digestion,  and  it  was  formerly  considered 
that  the  starch  arose  directly  from  the  oil.  It  appears 
now  that  the  oil  is  split  up  by  an  enzyme,  lipase,  the  result 
being  the  formation  of  a  free  fatty  acid  and  glycerine.  The 
subsequent  decompositions  are  very  complex,  among  the 
products  being  lecithin,  a  peculiar  fatty  substance  containing 
phosphorus,  as  well  as  several  simpler  acid  bodies,  which 
are  crystalline  instead  of  being  viscid  like  the  fatty  acid 
first  liberated.  These  pass  into  the  general  body  of  the 
seedling.  The  glycerine  appears  to  contribute  to  the  forma- 
tion of  the  lecithin.  The  decomposition  is  accompanied  by 
the  appearance  of  sugars  and  starch,  which  are  probably 
formed  by  the  protoplasm  of  the  cells. 

The  production  of  alcohol  from  sugar  is  brought  about  by 
another  soluble  enzyme,  which  has  been  prepared  from 
yeast.  Like  the  decomposition  which  is  brought  about  by 
myrosin,  the  splitting  up  of  the  sugar  is  apparently  not  a 
process  of  hydrolysis.  It  may  be  expressed  by  the  following 
equation  : 

C6H1206  =  2C02  4-  2CH3CH2OH. 

In  the  reaction  the  sugar  is  decomposed,  alcohol  is  formed 
and  carbon  dioxide  given  off. 

This  enzyme,  which  has  been  called  zymase,  has  been 


DIGESTION  261 

proved  to  exist  not  only  in  yeast,  but  in  certain  fruits,  being 
formed  there  when  the  fruits  are  kept  in  an  atmosphere 
which  contains  no  oxygen. 

The  physiological  explanation  of  this  observation  will 
be  discussed  more  fully  in  a  subsequent  chapter. 

There  are  other  enzymes  with  a  more  restricted  distri- 
bution, about  whose  value  to  the  plant  little  or  nothing  is 
known  at  present.  The  cells  of  a  particular  microscopic 
organism,  known  as  Micrococcus  urece,  decompose  urea 
with  the  formation  of  ammonium  carbonate,  and  an  enzyme, 
urease,  having  the  same  power,  can  be  extracted  from 
them.  Many  enzymes  can  be  prepared  from  bacteria, 
which  set  up  various  changes  in  proteins,  some  resulting  in 
the  formation  of  peptone,  and  others  producing  toxic  sub- 
stances. Many  bacteria  excrete  a  variety  of  diastase. 

Enzymes  of  another  class  do  not  apparently  take  any 
part  in  digestion,  but  may  be  briefly  alluded  to  here.  They 
set  up  a  process  of  oxidation  in  the  substances  they  attack, 
and  have  consequently  been  named  oxidases.  They  are 
apparently  very  widely  distributed,  and  perform  very 
various  functions,  being  often  concerned  in  bringing  about 
the  presence  of  particular  colouring  matters.  They  occur 
very  prominently  in  Fungi,  but  are  by  no  means  confined 
to  them.  Others  of  a  similar  character  act  exactly  oppositely 
and  have  been  called  reductases.  None  of  the  members 
of  either  class  have  at  present  been  very  fully  studied 
from  the  point  of  view  of  their  utility  to  the  plants  which 
secrete  them. 

The  conversion  of  zymogens  into  enzymes  is  much 
facilitated  by  a  gentle  warmth,  particularly  when  a  trace  of 
free  acid  is  present.  The  red  rays  of  light  exercise  a  similar 
influence  in  some  cases. 

The  fermentative  activity  of  protoplasm  was  alluded  to 
at  the  opening  of  this  chapter.  The  living  substance  of 
many  cells  is  capable  of  setting  up  various  fermentative 
decompositions,  apparently  identical  with  those  that  have 
been  described.  Various  cells  can  convert  starch  into 


262  VEGETABLE  PHYSIOLOGY 

sugar,  can  peptonise  proteins,  and  carry  out  other  digestive 
processes,  without  the  intervention  of  an  enzyme.  Though 
this  property  can  easily  be  proved  in  the  case  of  cells  of 
the  higher  plants,  it  is  especially  prominent  in  many  of  the 
more  lowly  organisms  such  as  the  Bacteria.  The  processes 
of  putrefaction  generally  depend  on  this  property  in  the 
organisms  which  bring  it  about.  Till  the  discovery  of 
zymase  the  alcoholic  fermentation  of  sugar  was  attributed 
to  such  an  action  in  the  yeast-cell,  and  in  the  cells  of  cer- 
tain ripe  fruits  under  particular  conditions,  the  chief  of 
which  was  the  deprivation  of  oxygen.  Such  an  action 
leads  to  the  formation  of  acetic  acid  from  alcohol  by  the 
microbe  Mycoderma  or  Bacterium  aceti.  Similar  proto- 
plasmic action  is  responsible  for  the  production  of  various 
acids  in  the  cells  of  the  higher  plants.  The  dependence 
of  these  fermentations  on  the  vital  activity  of  the 
protoplasm  is  evident  from  the  fact  that  no  enzyme  can 
be  extracted  from  the  cells  which  can  set  up  the  particular 
changes  in  question. 

It  is  not  difficult  to  prepare  the  enzymes  from  the  tissues 
in  which  they  work,  but  it  would  be  extremely  rash  to 
say  that  they  are  in  anything  like  a  pure  condition  when 
obtained.  Nor  is  it  easy  to  say  much  about  the  purifica- 
tion, as  they  are  not  known  except  in  close  connection  with 
the  substances  on  which  they  act,  or  with  the  products  of 
the  decompositions  they  initiate.  There  is  therefore  no 
known  test  of  their  purity. 

They  can  be  extracted  by  treating  the  tissue,  which 
should  be  very  finely  divided  or  ground  in  a  mortar,  with 
glycerine,  or  with  a  solution  of  common  salt,  or  with  water 
containing  a  trace  of  an  antiseptic.  After  a  period  of  ten 
or  twelve  hours  the  extract  should  be  strained  and  subse 
quently  filtered,  when  the  enzyme  may  be  precipitated  from 
the  filtrate  by  adding  strong  alcohol.  It  is  very  evident 
that  this  process  will  not  yield  it  pure,  for  the  solvents 
employed  will  dissolve  many  constituents  of  the  tissue 
besides  the  enzymes,  particularly  proteins  and  sugars. 


DIGESTION  263 

The  former  will  be  thrown  down  with  the  enzyme  by  the 
alcohol.  Moreover,  some  enzymes  are  slightly  soluble  in 
mixtures  of  alcohol  and  water  of  varying  degrees  of  con- 
centration, while  alcohol  destroys  others. 

Any  description  of  the  process  of  digestion  should  natu- 
rally be  followed  by  an  account  of  the  subsequent  one  of 
true  assimilation,  or  the  construction  of  protoplasm  from 
the  food  which  is  supplied  to  it  as  the  result  of  digestion. 
Unfortunately  but  little  can  be  said  upon  this  subject,  as  such 
problems  remain  almost  entirely  unsolved.  If  we  study  the 
changes  which  take  place  in  the  growing  points  of  plants, 
where  such  assimilation  must  necessarily  be  most  active,  we 
can  find  very  little  evidence  of  what  is  taking  place.  We  can 
trace,  for  instance,  the  progress  of  sugar  along  the  stem  for 
a  considerable  distance,  but  just  where  it  is  assimilated  our 
methods  fail  us.  Sugar  can  no  longer  be  detected,  but  in 
what  way  it  has  been  incorporated  into  the  living  substance 
is  still  a  mystery.  Similar  acknowledgment  must  be  made 
in  respect  of  the  proteins.  Amido-acids  can  be  detected 
along  the  translocatory  paths  almost  up  to  the  locality  of 
growth,  but  beyond  that  nothing  can  at  present  be  said 
with  certainty.  It  may  be  that  sugar  can  be  made  to  combine 
with  the  molecule  of  protoplasm,  and  that  amino-  or  amido- 
acids  and  not  protein  are  the  nitrogenous  materials  that 
are  actually  incorporated  into  the  living  substance.  We 
are  unable  also  to  explain  the  manner  in  which  the  food 
originally  constructed  ministers  to  the  nutrition  of  the 
protoplasts  or  cells  in  which  it  is  formed. 


264  VEGETABLE  PHYSIOLOGY 


CHAPTEK  XVII 

METABOLISM 

We  have  seen  that  the  object  of  all  the  processes  of  con- 
struction and  digestion  that  we  have  examined  so  far  has 
been  to  present  to  the  protoplasm  materials  which  it  can 
incorporate  into  its  own  substance.  If  we  consider  the 
processes  which  take  place  in  a  vegetable  cell  or  protoplast, 
we  find  that  they  can  be  divided  into  those  which  minister 
to  this  construction  or  building  up  of  the  living  substance, 
and  those  which  are  connected  with  its  breaking  down.  The 
latter  accompany  or  immediately  follow  the  former,  and  the 
two  together  may  be  considered  as  the  manifestation  of 
the  life  of  the  protoplasm.  The  whole  round  of  changes  in 
which  the  living  substance  is  concerned  is  generally  spoken  of 
as  its  metabolism.  So  many  of  the  reactions  as  culminate  in 
the  construction  of  protoplasm  are  described  as  anabolic, 
while  the  changes  which  it  initiates,  or  which  are  concerned 
in  its  decomposition,  are  termed  katabolic. 

We  have  been  occupied  mainly  so  far  in  discussing  the 
anabolism  of  the  protoplasts.  The  substances  we  have 
traced  to  the  cells  in  which  growth  and  repair  are  vigorous 
consist  in  far  the  greatest  part  of  some  form  of  sugar  and 
of  organic  nitrogenous  substances,  either  proteins  them- 
selves or  the  products  of  their  decomposition,  or  substances 
constructed  from  simple  materials  with  a  view  to  the 
formation  of  proteins,  such  as  asparagin  or  leucin.  In  the 
anabolic  processes  the  protoplasm  is  continually  recon- 
structing itself  at  the  expense  of  such  nutritive  substances, 


METABOLISM  265 

which  indeed  constitute  its  food  in  the  strict  sense  of  the 
term.  What  is  true  of  such  cells  as  are  actively  growing 
andjinultiplying,  which  are  found,  as  we  have  seen,  in  the 
special  growing  points  or  layers,  is  equally  true  of  all  cells~ 
so  long  as  they  are  living.  In  all  cases,  though  growth 
and  division  may  not  be  evident,  we  have  to  do  with 
processes  of  repair  of  the  inevitable  wasting  of  the  living 
substance  during  the  operations  of  its  life.  The  same 
kind  of  change  is  evident  in  all  cells,  though  the  immediate 
results  of  such  changes  differ  according  to  the  part  any 
particular  cell  takes  in  promoting  the  well-being  of  the 
whole  organism. 

If  we  turn  from  these  anabolic  processes  we  mid  we 
have  proceeding,  side  by  side  with  them,  a  decomposition 
of  the  protoplasm,  involving  a  separation  from  its  complex 
molecule  of  various  substances  which  are  of  less  com- 
plexity than  the  living  material  itself.  These  often,  in 
the  first  instance,  include  such  carbohydrates  and  nitro- 
genous substances  as  it  made  use  of  in  building  itself  up. 
These  can  again  be  used  in  reconstruction  of  the  proto- 
plasm, or  can  be  further  broken  down  into  simpler  substances 
still  or  can  be  retained  unaltered.  So  long  as  the  proto- 
plasm is  living,  it  is  continually  undergoing  constant 
reconstruction  and  decomposition. 

Besides  initiating  those  chemical  changes  in  which  it 
takes  this  prominent  part,  it  is  also  the  seat  of  a  large 
number  of  others  into  which  its  own  molecule  does  not 
immediately  enter.  Processes  of  both  oxidation  and  reduc- 
tion are  continually  going  on  in  its  substance,  in  which  are 
involved  the  various  materials  which  are  found  there,  either 
in  solution  in  the  water  which  saturates  it,  or  in  amorphous 
form  ;  substances  which  have  been  transported  from  other 
cells,  or  have  been  formed  in  the  processes  of  the  self- 
decomposition  of  the  protoplasm. 

Two  classes  of  enzyme  have  been  discovered  which 
may,  and  probably  do,  assist  in  these  changes.  They  are 
the  oxidases,  to  which  allusion  has  been  already  made, 


266  VEGETABLE  PHYSIOLOGY 

and  the  reductases,  which  act  in  the  opposite  direction.  The 
former  have  been  known  for  some  time,  the  latter  have 
been  observed  only  recently. 

The  katabolic  processes  vary  a  great  deal  in  the  extent 
to  which  they  are  carried  out.  They  may  sometimes  go 
on  so  far  as  to  produce  such  simple  bodies  as  carbon  dioxide 
and  water,  which  are  given  off  from  the  organism.  This 
is  a  very  marked  feature  of  the  metabolism  that  may  be 
observed  in  every  living  cell.  Other  katabolic  changes, 
proceeding  side  by  side  with  this  very  complete  decom- 
position, are  not  so  far-reaching,  and  a  great  accumula- 
tion of  their  products  remains  in  the  plant.  Prominent 
among  them  we  find  the  cell-walls  of  woody  or  corky 
tissue.  These  must  not  be  confused  with  what  we  have 
described  as  reserve  materials,  as  the  latter,  unlike  those 
now  under  discussion,  are  intended  for  ultimate  consumption. 

These  changes  involve  the  manufacture  of  great 
masses  of  material,  whose  construction,  though  ultimately 
dependent  upon  anabolism,  is  essentially  a  mark  of  the 
katabolic  processes.  The  constructive  processes  indeed 
are  both  anabolic  and  katabolic,  the  former  culminating 
in  the  formation  of  living  substances,  the  latter  marking 
the  fabrication  of  its  products.  The  great  extent  to 
which  the  constructive  katabolic  processes  exceed  such 
decomposition  of  protoplasm,  as  is  marked  by  the  forma- 
tion of  carbon  dioxide  and  water,  finds  its  expression  in 
the  enormous  bulk  which  many  trees  and  other  plants 
attain.  This  increase  of  the  size  of  the  plant-body  is  very 
much  facilitated  by  the  fact  that  the  katabolic  processes 
in  question  are  not  attended  by  the  excretion  of  any- 
thing from  the  body  of  the  organism.  As  a  rule  plants 
have  no  excreta  except  the  gaseous  bodies  whose  elimination 
we  have  already  described,  and  these  result  in  the  main 
from  the  profounder  decomposition  of  the  living  substance. 
Whatever  a  plant  absorbs  from  the  soil,  except  water,  it 
nearly  always  retains  within  its  tissues,  so  that  increase  of 
weight  almost  inevitably  accompanies  continuance  of  vitality. 


METABOLISM  267 

It  must  nut  be  inferred,  however,  that  plants  do  not 
produce,  during  their  constructive  katabolic  processes,  any 
substances  which  are  useless  to  them  or  which  may  even 
be  deleterious.  There  are  numerous  products  which  come- 
under  this  category,  but  from  the  mode  of  construction 
of  the  body  of  the  plant  they  are  not  cast  off  as  they 
would  be  from  the  animal  organism  under  similar  conditions. 
Instead  of  being  eliminated  entirely  they  are  only  removed 
to  such  localities  as  ensure  their  being  withdrawn  from 
the  spheres  of  vital  activity.  They  are  generally  deposited 
in  such  regions  as  leaves  which  are  about  to  be  shed,  or 
the  bark  of  trees,  which  is  a  collection  mainly  of  dead 
matter ;  or  they  may  be  stored  away  in  special  cells,  or  in 
cell- walls,  or  intercellular  passages,  or  elsewhere.  These 
bodies  really  correspond  to  excreta,  and  the  processes  of 
their  formation  and  deposition  are  properly  looked  upon  as 
processes  of  excretion. 

Most  of  the  katabolic  constructive  processes  are  directly 
applied  to  the  production  of  substances  which  are  of  great 
use  to  the  plant.  Emanating  as  these  do  directly  from  the 
protoplasm,  their  formation  is  generally  termed  secretion. 

Though  they  originate,  however,  directly  in  and  from 
the  living  substance,  the  latter  does  not  always  present 
them  in  the  form  in  which  they  are  found  in  the  adult 
plant-body,  for  various  changes  both  of  the  nature  of  oxida- 
tion and  reduction  may  take  place  in  them  after  they  have 
been  secreted. 

The  processes  included  under  the  general  term  katabolism 
are  thus  seen  to  be  very  varied.  During  the  course  of  such 
changes  many  substances  are  frequently  formed  which 
seem  to  have  no  direct  bearing  on  the  vital  processes,  and 
whose  meaning  is  still  obscure.  These  are  often  spoken  of 
as  the  lye-products  of  metabolism. 

We  may  now  pass  to  consider  in  some  detail  some  of 
the  more  prominent  processes  of  secretion. 

For  many  reasons  the  formation  of  such  enzymes  as  are 
used  during  digestion  may  be  regarded  as  the  most  typical 


268  VEGETABLE  PHYSIOLOGY 

of  these.  A  cell  which  is  about  to  secrete  is  generally 
found  to  be  filled  with  colourless  hyaline  protoplasm  in 
which  certain  vacuoles  may  be  seen.  Immediately  before 
secretion  begins  an  increase  of  the  amount  of  the  protoplasm 
can  be  observed,  which  is  effected  at  the  expense  of  various 
nutritive  products  which  are  transported  to  it.  During  the 
whole  of  the  process,  when  this  is  prolonged,  such  a  supply 
of  nutritive  material  takes  place.  If  during  the  secretion  this 
supply  is  stopped,  the  process  is  rapidly  suspended.  This 
can  be  detected  easily  in  the  case  of  the  epithelium  of  the 
scutellum  of  the  barley  grain,  which  we  have  seen  produces 
considerable  quantities  of  diastase.  The  first  stage  of  the 
process  is  thus  evidently  anabolic.  As  soon  as  the  nutrition 
of  the  cell  has  reached  a  certain  point  the  appearance  of 
the  protoplasm  undergoes  a  change.  Minute  granules 
begin  to  be  formed  in  its  substance,  which  increase  in 
number  until  the  hyaline  character  is  replaced  by  a 
marked  uniform  granularity,  the  cell  substance  becoming 
somewhat  like  ground-glass  in  appearance.  The  growth 
of  the  protoplasm  and  this  subsequent  formation  of  granules 
lead  to  the  obliteration  of  the  vacuoles,  till  the  cell  is  com- 
pletely filled.  After  a  time  as  the  secretion  leaves  the  cell 
the  latter  shrinks  again  ;  the  granules  are  passed  out  in 
solution  in  the  sap  which  is  exuded,  and  the  protoplasm  is 
seen  to  be  less  plentiful  and  to  become  hyaline  and  vacuo- 
lated  as  at  first. 

Following  the  anabolic  changes  we  have  thus  the  breaking 
down  of  the  protoplasm,  attended  by  the  appearance  of 
the  granules  to  which  it  has  given  rise.  There  is  reason 
to  believe  that  the  granules  consist  of  the  zymogen  rather 
than  the  enzyme,  and  that  the  final  transformation  of 
the  former  into  the  latter  takes  place  just  as  the  exuda- 
tion of  the  sap  occurs. 

In  glands  in  which  the  process  of  secretion  is  repeated 
more  than  once,  similar  changes  may  be  traced.  The 
secretion  of  the  enzyme  in  these  cases  can  be  shown  to 
take  place  by  successive  stages.  The  preliminary  hyaline 


METABOLISM  269 

condition  is  followed  by  the  granular  one,  and  in  this  state 
the  cell  can  remain  for  some  time  before  the  enzyme  is 
discharged.  When  this  has  happened  the  hyaline  condition 
is  resumed. 

The  formation  of  the  cell-wall  which  separates  the  cells 
is  due  to  a  similar  activity  of  the  protoplasm.  The  division 
of  cells  or  the  development  of  new  protoplasts  will  be  more 
fully  considered  in  a  subsequent  chapter.  It  will  suffice  to 
say  here  that  in  all  ordinary  growing  points  this  division 
of  a  protoplast  into  two  is  followed  immediately  by  the 
formation  of  a  new  supporting  membrane  between 
them. 

The  division  of  the  cell  is  preceded  by  the  division  of 
its  nucleus,  which  is  attended  by  a  series  of  complicated 
movements  of  particular  constituents  of  its  substance.  The 
two  daughter-nuclei  are  situated  at  some  little  distance  from 
each  other  and  are  connected  by  a  number  of  delicate  fila- 
ments which  are  gathered  to  a  point  at  each  end  and  spread 
out  in  the  centre,  forming  what  is  called  the  nuclear  spindle. 
This  generally  stretches  completely  across  the  long  diameter 
of  the  cell. 

During  these  introductory  changes  the  hyaline  proto- 
plasm becomes  more  granular,  and  the  granules,  technically 
spoken  of  as  microsomata,  are  attracted  to  the  spindle 
fibres.  They  pass  along  these  fibrils  from  both  regions  of 
the  cell  and  form  a  plate  of  extreme  tenuity  across  it, 
midway  between  the  two  nuclei.  This  plate  soon  under- 
goes a  transformation,  the  granules  disappearing  and  the 
membrane  becoming  translucent,  and  so  forming  the  ordinary 
substance  of  the  cell-membrane,  generally,  though  perhaps 
not  strictly  accurately,  known  as  cellulose.  The  cell-wall 
is  thus  seen  to  be  formed  from  the  protoplasm,  or  to  be 
secreted  by  it,  the  granules  or  microsomata  of  which  it  is 
at  first  composed  being  the  result  of  decompositions  set  up 
in  the  living  substance. 

When  cell- walls  are  growing  in  thickness  or  in  surface 
a  similar  decomposition  of  the  protoplasm  can  be  observed. 


270  VEGETABLE  PHYSIOLOGY 

The  microsomata  or  granules  are  formed  in  the  proto- 
plasm and  are  gradually  deposited,  often  in  oblique  rows, 
upon  the  original  membrane.  They  are  subsequently 
changed  both  in  appearance  and  in  nature,  and  become  the 
first  thickening  layer  of  cellulose.  The  occurrence  of  the 
rows  of  granules  frequently  leads  to  the  striated  appearance 
which  can  be  noticed  on  the  walls  of  many  fibres,  particu- 
larly those  of  the  bast  of  the  fibro-vascular  bundles. 

In  all  cases  therefore  the  formation  of  cellulose  can  be 
traced  to  the  self-decomposition  of  the  protoplasm,  though 
whether  the  granules  are  actually  cellulose  or  an  inter- 
mediate substance  is  still  uncertain. 

A  very  similar  phenomenon  is  observable  in  the  forma- 
tion of  starch  grains.  In  this  case,  as  we  have  seen,  we  may 
either  have  to  deal  with  the  general  protoplasm  of  the  cell, 
or,  as  is  usual  in  reservoirs  and  in  ordinary  leaf  parenchyma, 
with  a  definite  plastid,  either  a  chloroplast  or  a  leucoplast. 
These  structures,  however,  may  be  regarded  as  specially 
differentiated  protoplasmic  bodies.  We  have  already 
discussed  their  behaviour  and  the  formation  of  the  starch 
grain  by  them.  Building  themselves  up  at  the  expense  of 
sugar  and  probably  of  various  nitrogenous  compounds, 
either  brought  to  them  or  remaining  in  their  substance, 
they  break  down  again  to  a  certain  extent,  splitting 
off  a  quantity  of  starch,  which  is  deposited  in  the  interior 
of  the  plastid,  sometimes  at  one  point,  sometimes  at 
several.  As  the  process  goes  on,  successive  laminae  or 
shells  of  starch  are  continually  deposited  round  the  original 
grain  or  granule  till  the  structure  of  the  fully  formed  starch 
grain  is  reached.  In  this  case  the  process  is  somewhat 
clearer  than  the  corresponding  one  in  that  of  cellulose,  as 
there  is  little  doubt  that  each  shell  is  composed  of  starch 
at  the  moment  of  its  deposition. 

The  formation  of  starch  is  in  these  cases  a  secretion  by 
the  plastid,  just  as  that  of  cellulose  is  a  secretion  by  the 
protoplasm  of  the  cell.  The  formation  of  the  small  starch 
grains  by  the  general  protoplasm  of  cells,  in  which  no 


METABOLISM  271 

plastid  is  present  is  brought  about  in  the  same  way,  though 
it  is  not  so  long  continued  and  the  formation  of  successive 
laminae  does  not  take  place. 

The  method  by  which  aleurone  grains  arise  in  the  meshes" 
of  the  cytoplasm  is  of  a  precisely  similar  character. 

The  formation  of  fat  is  due  to  the  same  kind  of  behaviour 
on  the  part  of  the  protoplasm.  It  can  be  observed  most 
easily  in  the  case  of  certain  fungi  when  they  are  living  under 
such  conditions  as  prevent  their  being  properly  nourished. 
The  protoplasm  in  the  hyphaa  diminishes  in  quantity,  the 
vacuolation  becomes  considerable,  and  their  cavities  are 
found  to  contain  large  drops  of  oil.  In  the  cells  of  seeds 
such  as  those  of  the  castor-oil  plant,  in  which  large  quantities 
of  oil  are  stored  as  reserve  materials,  the  deposition  of  fat 
can  be  studied.  Sections  of  the  cells  should  be  stained 
with  osmic  acid,  which  colours  fatty  substances  brown  or 
black  according  to  the  quantity  of  them  which  is  present. 
When  fat  is  beginning  to  be  formed  the  substance  of  the 
protoplasm  becomes  faintly  granular,  but  the  granules  are 
more  or  less  transparent  and  cannot  be  seen  without 
staining.  On  the  application  of  osmic  acid  they  display 
their  fatty  character  by  becoming  brown.  In  cells  which 
are  a  little  older  the  granularity  is  more  marked,  as  the 
separate  granules  have  increased  in  size  and  many  have 
run  together,  forming  small  droplets.  The  staining  is 
darker  in  these  cells,  the  larger  granules  becoming  quite 
black.  In  still  older  cells  the  whole  substance  becomes 
intensely  black  and  the  protoplasm  can  be  seen  to  be 
saturated  with  the  oil.  If  the  latter  is  removed  by  treat- 
ment with  ether,  the  living  substance  is  found  to  have 
diminished  in  amount.  There  has  been  a  formation  of 
fat  by  the  protoplasm,  and  the  latter  has  evidently  pro- 
duced it  by  a  decomposition  of  its  own  substance,  for  it  has 
become  reduced  in  bulk. 

The  elai'oplasts,  to  which  reference  has  been  made, 
behave  similarly,  their  substance  diminishing  at  the  same 
time  that  the  fat  or  oil  makes  its  appearance. 


272  VEGETABLE  PHYSIOLOGY 

The  decomposition  of  the  protoplasm  in  the  formation 
of  fat  is  not  accompanied  by  much  reconstruction,  so  that  it 
is  soon  very  greatly  diminished  in  amount,  while  the  fat, 
the  product  of  the  katabolic  processes,  increases. 

The  appearance  of  fat  in  the  two  cases  described  seems 
to  demand  two  different  explanations.  In  the  cells  of  the 
seeds  and  in  the  ela'ioplasts  it  is  to  be  regarded  as  a 
storage  of  reserve  materials.  In  the  starved  hyphse  of 
the  Fungus  it  appears  to  be  due  to  the  decomposition 
of  protoplasm  under  conditions  of  grave  disturbance  of 
nutrition,  if  not  of  approaching  death.  In  both  cases, 
however,  it  is  derived  from  the  breaking  down  of  the  living 
substance,  though  the  decomposition  of  the  latter  is  due 
to  such  different  causes  in  the  two  cases. 

One  of  the  most  important  of  the  secretions  of  plants  is 
the  green  colouring  matter,  chlorophyll,  which  we  have 
already  seen  is  present  in  the  form  of  a  solution  in  the 
meshes  of  the  chloroplasts.  The  formation  of  chlorophyll 
is  a  more  specialised  process  than  any  of  those  which  we 
have  just  been  considering,  and  is  dependent  upon  a 
variety  of  conditions.  It  probably  involves  not  only  the 
self -decomposition  of  the  protoplasm,  but  also  other 
processes  which  take  place  within  the  substance  of  the 
chloroplast. 

The  special  conditions  necessary  for  the  formation  of 
chlorophyll  are — 1st,  access  of  light ;  2nd,  a  particular 
range  of  temperature ;  3rd,  the  presence  of  a  minute 
quantity  of  iron  in  the  plant ;  4th,  access  of  oxygen.  There 
are  a  few  exceptions  to  the  rule  that  chlorophyll  can  be 
formed  only  in  light  ;  the  embryo  in  the  seed  of  Euonymus 
europceus  is  green  at  the  time  the  seed  is  ripe,  though  it  is 
surrounded  by  a  thick  red  protecting  coat  which  is  opaque. 
Seedlings  of  Pinus  also  are  green  when  they  are  raised 
from  seeds  in  light  which  is  insufficiently  strong  to  enable 
chlorophyll  to  be  formed  in  seedlings  of  Dicotyledons  grown 
side  by  side  with  them.  A  few  other  cases  also  are  known. 
If  an  ordinary  plant  is  cultivated  from  the  seed  in  darkness, 


METABOLISM  273 

the  resulting  seedling  will  not  be  green,  but  will  have  a 
yellowish-white  colour.  When  its  tissues  are  examined 
with  a  microscope,  the  plastids  will  be  found  in  the  cells, 
but  they  will  be  tinged  with  a  pale  yellow  pigment  known 
as  etiolin.  This  is  in  the  first  instance  secreted  by  the 
protoplasm  of  the  plastid,  and  subsequent  changes  take 
place  about  which  very  little  is  known,  but  which  result  in 
its  replacement  by  chlorophyll.  If  the  temperature  is  kept 
very  low,  the  etiolin  remains  unchanged,  even  though  light 
is  admitted.  Hence  the  first  leaves  of  plants  which  spring 
up  in  winter  or  early  spring  are  frequently  yellow  and  not 
green.  This  peculiarity  may  easily  be  observed  in  the  case 
of  snowdrops  and  hyacinths  which  appear  very  early  in  the 
year. 

The  function  of  the  iron  is  not  understood  ;  plants  which 
are  cultivated  in  such  a  medium  that  this  element  is  not 
supplied  to  them  have  an  appearance  much  like  that 
associated  with  etiolation.  They  are'  indeed  almost  colour- 
less, though  the  plastids  are  present.  A  supply  of  iron  at 
once  causes  them  to  assume  the  normal  appearance.  Plants 
so  suffering  from  the  absence  of  iron  are  said  to  be 
chlorotic. 

The  influence  of  a  supply  of  oxygen  is  probably  not  a 
direct  one.  The  failure  of  plants  to  form  chlorophyll  in 
its  absence  is  most  likely  due  to  a  pathological  or  unhealthy 
condition  of  the  protoplasm,  all  whose  activities  are  dis- 
turbed under  such  circumstances. 

Another  pigment  which  is  of  fairly  widespread  distri- 
bution in  plants  is  the  red  or  purple  colouring  matter  known 
as  anthocyan.  This  is  not  associated  with  any  plastids,  but 
occurs  in  solution  in  the  cell-sap,  particularly  of  the  cells  of 
petals  and  some  other  parts  of  flowers  ;  it  is  found  also  very 
commonly  in  young  developing  shoots,  on  the  illuminated  side 
of  leaves  which  appear  during  cold  weather,  on  the  petioles 
and  midribs  of  leaves  which  are  put  out  on  twigs  of  many 
plants  in  sunny  places,  and  in  many  tropical  plants  which 
grow  in  deep  shade.  In  seedlings  which  are  developed  in 

18 


274  VEGETABLE  PHYSIOLOGY 

spring  or  in  cold  weather,  the  anthocyan  may  appear  some- 
what irregularly  in  the  leaves,  but  it  is  found  mainly  along 
the  veins  and  on  the  leaf-stalk.  It  is  probable  that  the 
colour  is  produced  by  the  oxidation  of  an  antecedent  colour- 
less aromatic  substance. 

The  function  of  anthocyan  is  not  well  understood. 
Many  facts  point  to  the  probability  that  it  aids  in  the 
transformation  of  starch  into  sugar  in  the  leaves  in  which 
it  occurs,  rendering  translocation  more  rapid.  Engelmann 
has  showed  that  it  absorbs  the  rays  of  light  complementary 
to  those  absorbed  by  chlorophyll.  It  has  been  found 
that  the  red  rays  of  the  solar  spectrum  which  it  allows  to 
pass  are  instrumental  in  the  formation  of  leaf -diastase  from 
its  antecedent  zymogen.  The  pigment,  while  allowing 
these  red  rays  to  pass  into  the  leaf,  acts  as  a  screen  preventing 
the  passage  of  the  violet  ones,  which  have  a  very  destructive 
effect  upon  this  enzyme. 

Other  views  as  to  the  significance  of  this  pigment  have 
been  advanced.  It  has  been  suggested  that  it  effects  a 
conversion  of  light  rays  into  heating  ones,  so  facilitating 
the  metabolic  processes  of  the  plant.  Another  hypothesis 
regards  it  as  a  protective  screen  to  the  chloroplasts  and  to 
the  protoplasm,  preserving  them  from  injury  from  too 
intense  light.  Neither  of  these  views  can,  however,  be 
regarded  as  entirely  satisfactory. 

In  many  cases  it  acts  beneficially  by  absorbing  the  dark 
heat  rays  and  so  facilitating  transpiration  as  well  as  general 
metabolism. 

Anthocyan  appears  to  be  a  derivative  of  tannin,  an 
aromatic  substance  which  is  very  widely  distributed  in 
the  vegetable  organism.  This  substance  has  not  generally 
been  included  among  the  secretions  of  plants,  but  rather 
as  a  bye-product  of  metabolism.  It  is  not  impossible  that 
it  may  in  some  cases  be  a  definite  secretion  for  some 
particular  purpose. 

The  distinction  between  definite  processes  of.  secretion 
and  such  reactions  as  lead  to  the  formation  of  the  so-called 


METABOLISM 


275 


bye-products  of  metabolism  is  not  at  all  well  defined.  In 
many  cases  substances  are  included  in  the  latter  category 
because  nothing  is  known  as  to  their  function,  arid  the 
classification  can  therefore  be  regarded  only  as  provisional. 
In  many  cases  it  cannot  yet  be  determined  whether 
particular  substances  are  formed  by  the  direct  decomposi- 
tion of  protoplasm,  or  by  subsequent  changes  in  the  primary 
products  of  such  decomposition.  Till  quite  recently  the 
formation  of  resin  and  allied  bodies  in  the  resin  passages 
of  the  Conifers  and  in  many  glandular  hairs  was  con- 
sidered a  true  secretion,  the  aromatic  substances  being  held 


FIG.   124. — GLANDULAR   HAIRS    FROAI 
Primula  sinensis. 

a,  young  hair;  b,  hair  showing  secre- 
tion formed  in  the  cell-wall  of  the 
terminal  cell ;  c,  hair  after  discharge 
of  the  secretion. 


FIG.    125. — GLANDULVR   HAIIIS    FROM 

THE   HOP. 

A,  young  hair  ;  B,  mature  hair  ; 
5.C.,  secretion  under  the  cuticle. 


to  arise  in  the  cells.  Recent  investigations  tend  to  show  that 
this  is  not  the  mode  of  their  origin  at  all,  but  that  these 
substances  are  formed  by  a  peculiar  process  of  degradation 
of  the  cell- wall.  The  glandular  hairs  of  Primula  sinensis 
(fig.  124),  and  the  more  complex  one  of  the  Hop  (fig.  125), 
have  long  been  known  to  form  their  resins  in  this  way.  It 
seems  probable  that  we  must  now  regard  the  resin-secret- 
ing organs  of  the  Conifers  as  comparable  with  these. 

The  bye-products  of  metabolism  are  too  numerous  to 
be  discussed  in  detail  in  the  present  treatise.  Though 
they  seem  to  be  quite  subordinate  to  the  main  products  we 
have  noticed,  and  to  be  formed  indeed  by  decompositions 

18* 


276  VEGETABLE  PHYSIOLOGY 

which  take  place  during  the  construction  of  the  latter,  we 
should  not  be  warranted  in  ignoring  their  possible  utility 
to  the  plant,  nor  the  probability  that  many  of  them  may 
be  of  nutritive  value.  We  have  seen  that  in  the  decom- 
position of  amygdalin  by  its  appropriate  enzyme  emu!  sin, 
besides  the  undoubtedly  nutritive  sugar  there  is  a  produc- 
tion of  prussic  acid  and  benzoic  aldehyde.  Borne  plants 
have  been  shown  to  be  capable  of  utilising  the  former  of 
these,  toxic  as  it  is  to  many  forms  of  animal  life. 

The  bye-products  include  bodies  of  very  varying  degrees 
of  complexity,  some  nitrogenous  and  others  not.  Among 
the  former  may  be  mentioned  the  great  group  of  the 
alkaloids,  many  of  which  have  not  so  far  been  found  able 
to  minister  to  the  nutrition  or  growth  of  the  plant,  though 
their  nitrogen  is  in  organic  combination.  If  a  plant  is 
supplied  with  them,  but  with  no  other  form  of  combined 
nitrogen,  it  is  rapidly  starved.  In  certain  cases  in  which 
relatively  large  quantities  of  them  are  stored  in  seeds,  they 
have  been  observed  to  diminish  in  amount  during  germi- 
nation. They  may  have  a  nutritive  value  in  these  cases. 
Many  physiologists  consider  this  group  to  belong  rather  to 
the  definite  excretions  of  the  plant  than  even  to  its  bye- 
products.  They  are  usually  deposited  in  regions  which  are 
situated  well  away  from  the  seats  of  active  life,  such  as  the 
bark  of  trees,  the  pericarps  of  fruits,  &c.  It  is  apparently 
very  difficult  to  draw  a  distinct  line  of  separation  between 
excretions  and  bye-products,  just  as  it  is  to  distinguish 
clearly  between  the  latter  and  secretions. 

The  amidated  fatty  acids,  as  we  have  seen,  generally 
occur  in  direct  relation  to  nutrition.  We  have  examined 
the  part  played  by  leucin  and  asparagiri  in  protein  con- 
struction and  metabolism.  Several  other  related  substances 
are  met  with  in  various  plants,  but  how  far  they  are  avail- 
able for  nutrition  and  how  far  they  are  merely  bye-products 
is  uncertain.  Such  substances  are  xanthin  and  glycin, 
which  can  be  extracted  from  various  cells.  The  latex  of 
plants  frequently  contains  many  of  these  substances. 
Caoutchouc  is  also  a  frequent  constituent  of  latex. 


METABOLISM  277 

Among  the  non-nitrogenous  bye-products  may  be  mem- 
tioned  the  great  variety  of  vegetable  acids.  Conspicuous 
among  these  are  tartaric,  malic,  citric,  and  acetic  acids. 
They  are  usually  regarded  as  arising  in  the  course  of  the 
katabolic  processes,  but  it  is  at  least  possible  that  some 
of  them  may  be  formed  in  the  elaboration  of  food  from 
the  raw  materials  absorbed,  having  thus  their  origin  in 
anabolism. 

The  bye-products  include  also  a  variety  of  aromatic  sub- 
stances. Mention  has  already  been  made  of  tannin,  and 
its  position  discussed.  In  addition  we  may  include  phloro- 
glucin  and  a  variety  of  aromatic  acids,  such  as  benzoic, 
salicylic,  &c.,  but  the  nature  of  the  processes  which  give 
rise  to  them  is  not  well  ascertained. 

Certain  decomposition  products  of  cellulose  may  also  be 
mentioned  here.  The  lignin  and  suberin  which  are  char- 
acteristic of  woody  and  corky  cell-walls  arise  in  this  way. 
During  their  formation,  which  takes  place  in  the  substance 
of  the  cell- wall,  they  can  be  removed  by  appropriate  solvents, 
leaving  the  remainder  of  the  cellulose  skeleton  which  they 
have  been  gradually  replacing.  These  differ  from  most  of  the 
substances  described  in  that  they  can  be  produced  in  the 
walls  of  cells  that  have  lost  their  protoplasm,  so  that  their 
formation  is  not  directly  dependent  on  metabolism. 

We  have  again  the  odorous  substances,  and  the  colour- 
ing matters  other  than  those  already  mentioned.  Many 
colouring  matters  are  products  of  the  decomposition  of 
chlorophyll,  especially  certain  of  those  to  which  the  autumnal 
tints  of  leaves  are  due.  One  of  this  group,  xanthophyll,  is 
a  bright  yellow  pigment  which  is  always  associated  with 
the  chlorophyll,  though  in  varying  amount. 

We  have  finally  in  connection  with  the  metabolic  pro- 
cesses to  touch  upon  the  excretions  of  plants.  The  term 
must  be  used  in  a  wide  sense  to  include  all  such  substances 
as  are  undoubtedly  withdrawn  from  the  seats  of  active  life, 
whether  thrown  off  from  the  plant-body  or  not.  The 
excreta  which  are  completely  eliminated  are  few ;  under 
normal  conditions  only  the  carbon  dioxide  and  water  which 


278 


VEGETABLE  PHYSIOLOGY 


are  products  of  respiration  can  be  specified.  Under  abnormal 
conditions  volatile  compounds  of  ammonia  or  ammonia 
itself  may  be  added  to  these.  But  there  are  certain  other 
substances  which  are  thrown  off  by  a  few  plants,  and  may  in 
them  perhaps  be  regarded  rather  as  secretions,  as  some  of  them 
subserve  definite  purposes.  Perhaps  the  most  frequently 
occurring  instance  of  these  is  the  sugary  solution  known 
as  the  nectar,  which  is  so  common  in  flowers,  and  which  is 


Fia.  126. — DEVELOPMENT  OF  LYSIGENOUS  GLAND  IN 
STEM  OF  Hypericum.  THE  FOUR  FIGURES  REPRESENT 
SUCCESSIVE  STAGES,  x  250. 


FIG.    127.  — CRYS- 
TALS  OF  CALCIUM 

OXALATE      IN 

WALL  OF  CELL 
OF  THE  BAST  OF 
Ephedra. 


poured  out  usually  to  serve  as  an  attraction  to  insect  visitors. 
Mineral  matters  such  as  calcium  carbonate  are  in  some  cases 
excreted  on  to  the  surface  of  the  leaf,  sometimes  by  special 
glands,  as  in  certain  Saxifrages.  In  these  the  salt  aids  in 
the  formation  of  a  subsidiary  water-absorbing  apparatus, 
as  will  be  mentioned  in  a  subsequent  chapter. 

In  most  cases  the  materials  which  we  are  discussing  are 
not  thrown  off  from  the  plant,  but  are  removed  to  parts 
which  are  not  concerned  in  the  vital  processes  to  any  very 
great  extent.  Ethereal  oils  are  found  deposited  in  special 
cavities  in  leaves,  stems,  and  other  parts  (fig.  126).  Mineral 


METABOLISM 


279 


matters  are  often  deposited  in  the  substance  of  cell- walls. 
The  oxalate  of  calcium  occurs  frequently  in  this  situation 
(fig.  127).  In  other  cases  it  is  deposited  in  special  cells, 
where  it  forms  clusters  of  crystals  of  characteristic  shape 
(fig.  128,  A,  B).  In  these  cases  the  cluster  of  crystals  is 
usually  invested  by  a  delicate  skin  derived  from  the  proto- 
plasm, thus  shutting  it  off  completely  from  any  participa- 
tion in  the  metabolism  of  the  cell  in  which  it  lies. 

Carbonate  of  calcium  may  also  be  deposited  in  the  sub- 
stance of  the  cell-wall,  or  of  protrusions  from  it,  as  in  the 
cystoliths  of  Ficus,  Urtica,  and  other  plants  (fig.  129). 


FIG.  128. — CRYSTALS  OF  OXALATE  OF 
CALCIUM.  A,  FROM  BEET  (Sphcera- 
phides) ;  B,  FROM  ARUM  (Raphides). 


ej 


FIG.  129. — SECTION  OF  PORTION  OF 
LEAF  OF  Ficus,  SHOWING  CYSTO- 
LITH  (cys)  IN  LARGE  CELL  OF  THE 
THREE-LAYERED  EPIDERMIS  (ep). 
(pa)  PALISADE  LAYER, 


Silica  again  is  accumulated  in  the  epidermis  of  many 
grasses,  and  of  the  horsetails  (Equisetum). 

Though  many  of  these  substances,  both  excretions  and 
bye-products,  are  of  no  value  for  nutrition,  some  of  them 
may  play  a  very  important  part  in  the  defence  of  plants 
against  their  natural  enemies,  their  nauseous  smell  or 
flavour  preventing  their  being  eaten  by  animals,  &c.  Some 
odours  and  the  nectar  found  in  flowers  are  doubtless  of 
great  service  in  attracting  insects,  which  assist  in  the  process 
of  cross-pollination,  to  be  discussed  in  a  subsequent  chapter. 

Though  we  cannot  trace  the  formation  of  all  these  various 
substances,  both  bye-products  and  excretions,  directly  to 
the  self-decomposition  of  the  protoplasm,  but  must  regard 


280  VEGETABLE  PHYSIOLOGY 

them  as  formed  partly  by  the  processes  of  oxidation  and 
reduction,  which  we  have  seen  are  often  associated  with  its 
activity,  and  partly  by  subsequent  further  decompositions 
of  bodies  originally  thus  formed,  we  can  trace  them  ultimately 
to  protoplasmic  activity,  and  may  consequently  regard  their 
formation  as  belonging  to  the  katabolic  processes  going  on 
in  the  organism. 


281 


CHAPTEK  XVIII 

THE    ENERGY    OF    THE    PLANT 

The  various  operations  which  we  have  seen  to  be  con- 
tinually going  on  in  the  body  of  the  plant  involve  the  execu- 
tion of  a  considerable  amount  of  work.  This  is  very  evident 
when  we  observe  only  the  enormous  development  of  a 
large  tree,  and  compare  it  with  the  relatively  small  seed 
from  which  it  has  sprung.  Such  a  process  of  construction 
has  involved  the  preparation  of  a  vast  quantity  of  highly 
complex  material  from  very  simple  chemical  substances. 
The  processes  incident  to  life  also,  though  they  may  not 
lead  directly  to  the  formation  of  such  substances,  cannot 
be  conducted  without  involving  a  considerable  amount  of 
work,  whether  the  plant  is  a  minute  body  consisting  of  a 
single  protoplast,  or  an  organism  of  a  much  higher  degree 
of  complexity. 

We  must  therefore  turn  our  attention  to  the  question  of 
the  supply  and  utilisation  of  the  energy  at  the  expense  of 
which  the  various  processes  of  life  are  carried  out.  At  the 
outset  it  will  be  well  to  consider  what  demands  for  energy 
we  find  presented  by  the  plant,  or  what  are  the  ways  in 
which  energy  is  expended  or  lost. 

Some  of  these  have  been  incidentally  alluded  to  in  the 
preceding  chapters,  though  we  have  not  specially  regarded 
them  from  this  point  of  view.  We  may  refer  especially  to 
the  very  great  evaporation  of  water  from  the  living  cells 
into  the  intercellular  spaces,  which  we  have  seen  is  in  some 
cases  supplemented  by  an  evaporation  from  the  general 
external  surface,  when  this  is  not  covered  by  any  very 


282  VEGETABLE  PHYSIOLOGY 

distinct  cuticle.  It  is  evident  that  the  great  quantity  of 
water  which  is  given  off  by  the  leaves  of  a  sunflower,  to 
which  allusion  has  been  made  in  an  earlier  chapter,  cannot 
be  evaporated  without  the  expenditure  of  a  considerable 
amount  of  energy,  which  presumably  takes  the  form  of 
heat.  It  has  been  computed  recently  that  98  per  cent,  of 
the  energy  of  the  rays  of  light  which  are  absorbed  by  the 
chlorophyll  is  expended  in  causing  this  transpiration. 

The  great  accumulation  of  material  which  is  so  marked 
a  feature  of  the  life  of  a  plant  is  the  result  of  work  which 
has  been  carried  out  in  the  plant  on  the  simple  substances 
which  are  absorbed.  We  may  distinguish  here  between 
such  products  as  are  destined  for  immediate  or  ultimate 
consumption,  and  those  which  become  incorporated  into 
the  actual  substance  of  the  plant.  The  accumulation  of 
the  latter  is  permanent,  and  the  energy  which  is  used  in 
their  construction  is  not  subsequently  made  use  of  in  the 
working  of  the  organism.  That  it  is  stored,  however,  is 
evident  from  the  fact  that  it  can  be  re-converted  into 
heat  if  the  substance  is  burned.  As  we  shall  see  later,  the 
products  which  are  ultimately  consumed  in  the  nutritive 
processes  may  be  regarded  as  stores  of  energy  as  well  as  of 
nutritive  material.  In  both  cases,  however,  their  construc- 
tion involves  the  expenditure  of  a  considerable  amount  of 
energy  before  they  assume  their  recognisable  condition. 

Closely  allied  to  these  constructive  processes  we  have 
the  phenomena  of  repair  and  of  growth.  As  we  have  not 
yet  studied  the  latter  process  in  detail,  we  may  be  content 
with  pointing  out  that  there  are  involved  in  it  many  changes 
of  various  substances,  which  call  for  the  execution  of  con- 
siderable amount  of  work,  which  in  turn  demands  the 
expenditure  of  energy.  Many  organs  carry  out  their  growth 
under  conditions  of  pressure  :  roots,  for  instance,  in  their 
penetration  through  the  soil.  Not  only  is  energy  necessary 
to  produce  the  growth  itself,  but  the  pressure  upon  the 
growing  organs  must  be  counterbalanced  by  the  internal 
forces  they  exhibit, 


THE  ENEEGY  OF  THE  PLANT  283 

Many  of  the  humbler  plants  possess  a  considerable  power 
of  active  movement  or  locomotion.  Zoospores  of  many  of 
the  Algae  and  Fungi,  and  the  antherozoids  of  most  of  the 
other  Cryptogams  effect  this  locomotion  by  means  of  cilia 
which  wave  to  and  fro  vigorously  in  the  water  in  which  they 
find  themselves.  The  proportionate  amount  of  energy 
which  they  expend  in  this  way  is  very  great  compared  with 
the  total  amount  which  they  possess.  Other  movements 
which  are  not  dependent  upon  ciliary  action  are  not  un- 
common. The  amoeboid  movements  of  the  Myxomycetes 
or  slime  fungi,  the  rotation  and  circulation  of  the  sap  in 
many  cells,  the  other  internal  movements  of  protoplasm, 
the  movements  of  diatoms  and  the  oscillations  of  certain 
filamentous  Algse,  illustrate  these.  All  alike  are  dependent 
upon  a  certain  expenditure  of  energy. 

The  so-called  movements  of  the  growing  parts  of  plants 
are  frequently  quoted  in  this  connection.  As  we  shall  see 
hereafter,  however,  these  are  usually  'changes  of  position 
induced  by  variations  in  the  processes  of  growth,  and  may 
rather  be  referred  to  expenditure  of  energy  in  connection 
with  the  latter  than  to  actual  movement.  The  movements 
of  adult  organs  are  also  effected  by  causes  which  correspond 
in  great  measure  to  those  which  modify  growth,  being 
generally  brought  about  by  such  variations  in  the  turgescence 
of  particular  cells  or  groups  of  cells  as  those  upon  which  we 
shall  see  growth  largely  depends.  In  this  sense  they  are 
to  be  associated  with  modifications  of  the  hydrostatic 
tensions  in  the  parts  concerned.  A  certain  amount  of 
expenditure  of  energy  in  the  cells  concerned  is,  however, 
most  probable,  though  it  is  uncertain  how  far  such  changes 
as  modify  the  resistance  of  the  protoplasm  to  the  passage  of 
water  through  it  involve  the  application  of  energy.  The 
establishment  and  maintenance  of  the  turgid  condition, 
due  to  the  hydrostatic  distension  of  the  extensible  cell-wall, 
also  demands  the  expenditure  of  energy. 

We  have  instances  of  what  we  may  call  the  passive 
escape  of  energy  in  the  shape  of  heat,  and  to  a  less  extent 


284  VEGETABLE  PHYSIOLOGY 

in  the  manifestation  of  the  phenomena  of  so-called  phos- 
phorescence. Heat  is  lost  to  the  plant  in  many  ways,  one 
of  which,  the  evaporation  of  the  water  of  transpiration,  has 
already  been  mentioned.  Another  almost  equally  important 
source  of  loss  is  radiation  from  the  general  surface.  This 
is  greatest  from  flattened  members  of  the  plant,  such  as 
leaves. 

The  temperature  of  the  plant  is  very  largely  influenced 
by  that  of  the  air,  and  no  doubt  interchanges  of  heat  take 
place  in  both  directions.  But  it  must  not  be  concluded 
that  the  temperature  of  the  plant  and  that  of  the  air  always 
vary  together.  On  the  contrary,  radiation  under  some 
conditions  may  go  on  until  the  plant  is  several  degrees 
colder  than  the  surrounding  air.  This  is  probably  the 
explanation  of  the  ready  formation  of  dew  and  hoar  frost 
on  the  surfaces  of  leaves  at  certain  seasons  of  the  year. 
It  is  quite  of  frequent  occurrence  again  that  a  plant  or  part 
of  a  plant  may  have  a  much  higher  temperature  than  the 
air,  and  hence  a  copious  radiation  may  take  place.  During 
the  processes  of  germination  the  temperature  of  the  seed 
may  be  as  much  as  20°  C.  above  that  of  the  air.  The  open- 
ing of  flower  buds  is  also  attended  by  the  attainment  of  a 
high  temperature  and  a  consequent  escape  of  heat. 

If  we  turn  again  to  plants  with  a  watery  environment, 
the  loss  of  heat  may  be  observed  under  appropriate  con- 
ditions. It  is  well  known  that  the  processes  of  alcoholic 
fermentation  provoked  by  the  yeast-plant  are  attended  by 
the  liberation  of  heat,  which  is  given  off  by  the  active 
cells,  and  causes  a  considerable  rise  of  temperature  in  the 
fermenting  liquid. 

We  may  infer  also  from  a  consideration  of  the  various 
processes  we  have  studied,  and  from  the  fact  that  they  are 
carried  out  most  advantageously  within  a  certain  relatively 
small  range  of  temperature,  that  the  maintenance  of  such 
a  temperature  is  a  great  desideratum  to  the  plant.  There 
is  not  a  very  complete  mechanism  in  the  plant  to  secure 
this  object,  for  the  organism  generally  becomes  of  about 


THE  ENEKGY  OF  THE  PLANT  285 

the  same  temperature  as  the  medium  in  which  it  lives, 
though  the  process  of  adjustment  is  often  very  slow,  the 
tissues  being  generally  very  poor  conductors  of  heat.  Still 
it  seems  not  improbable  that  a  certain  amount  of  energy  is 
devoted  to  the  attainment  of  the  range  which  is  most  suit- 
able for  the  vital  processes.  Though  the  dominating  factor 
in  the  determination  of  the  plant's  temperature  is  to  be 
looked  for  in  the  environment,  the  development  of  heat 
during  germination  and  while  the  flower-bud  is  opening  is 
an  indication  that  it  is  not  the  only  one. 

A  fuller  consideration  of  the  relations  of  the  plant  to 
heat  must  be  deferred  to  a  subsequent  chapter. 

The  evolution  of  light  by  plants  is  a  comparatively  rare 
phenomenon,  being  probably  confined  to  certain  Fungi, 
though  it  has  been  attributed  also  to  a  few  species  of  Algae. 
It  must  call,  however,  for  a  certain  expenditure  of  energy 
in  such  cases  as  have  been  authenticated. 

If  we  turn  now  to  consider  the  sources  of  the  plant's 
energy,  it  is  evident  that  they  must  be  in  the  first  instance 
of  external  origin.  The  radiant  energy  of  the  sun  indeed 
is  the  only  possible  source  which  can  be  of  service  to  normal 
green  plants.  The  question  of  the  absorption  of  this  energy 
has  already  been  incidentally  alluded  to  when  we  discussed 
the  chlorophyll  apparatus,  but  it  may  now  be  examined 
more  closely. 

The  rays  which  emanate  from  the  sun  are  generally 
alluded  to  as  falling  into  three  categories,  those  of  the 
visible  spectrum,  those  of  the  infra-red,  and  those  of  the 
ultra-violet.  The  second  of  these  are  frequently  spoken  of 
as  heat  rays,  and  the  last  as  chemical. 

The  greatest  absorption  of  energy  appears  to  take  place 
in  consequence  of  the  peculiarities  of  chlorophyll.  As  we 
have  seen,  this  substance,  whether  in  the  plant  or  when  in 
solution  in  various  media,  absorbs  a  large  number  of  rays 
in  the  red  and  in  the  blue  and  violet  regions  of  the  spectrum, 
together  with  a  few  others  in  the  yellow  and  the  green. 

The  solar  spectrum  after  the  light  has  passed  through 


286 


VEGETABLE  PHYSIOLOGY 


a  solution  of  chlorophyll  is  seen  to  be  robbed  of  rays  in 
these  regions,  and  hence  to  present  the  appearance  of  a 
band  of  the  different  colours  crossed  by  several  dark  bands 
(fig.  130).  The  greater  part  of  the  energy  so  obtained  in 
the  cells  which  contain  the  chloroplasts  is  at  once  expended, 
partly  in  constructing  carbohydrate  food  materials  and 
partly  in  evaporating  the  water  of  transpiration.  The  latter 
process  is  much  the  more  expensive ;  recent  observations 
,  ave  made  it  probable  that  98  per  cent,  of  the  radiant  energy 


i  n 


FIG.  130. — ABSORPTION  SPECTRA  OF  CHLOROPHYLL  AND 
XANTHOPHYLL.    (After  Kraus.) 


actually  absorbed  during  bright  sunshine  is  at  once  devoted 
to  this  purpose. 

When  we  speak  of  radiant  energy  we  must  remember 
that  the  rays  of  the  visible  spectrum  do  not  supply  all  the 
energy  which  the  plant  obtains.  It  has  been  suggested  by 
several  botanists  with  considerable  plausibility  that  the 
ultra-violet  or  chemical  rays  can  be  absorbed  and  utilised 
by  the  protoplasm  without  the  intervention  of  any  pigment 
such  as  chlorophyll.  There  is  some  evidence  pointing  to 
this  power  in  the  cells  of  the  higher  plants.  Certain  bacteria 
also  construct  organic  material  from  simple  compounds  of 
nitrogen  and  carbon  dioxide,  though  it  is  not  probable  that 
they  utilise  radiant  energy  directly. 


THE  ENEEGY  OF  THE  PLANT  287 

Finally  we  have  evidence  of  the  power  of  plants  to  avail 
themselves  of  the  heat  rays.  The  relations  existing  between 
the  organism  and  its  environment  have  already  been  men- 
tioned. Not  only  can  the  air  rob  the  plant  of  heat  by 
radiation,  but  when  its  own  temperature  is  high  it  can  com- 
municate heat  to  it  in  turn.  Leaves  have  been  proved  to 
absorb  heat  with  great  avidity,  particularly  those  which 
are  succulent  or  fleshy,  a  difference  of  more  than  20°  C.  having 
been  noted  between  their  temperature  and  that  of  the  air. 
The  direct  absorption  of  the  rays  of  heat  from  the  sun  has 
also  been  noted,  apart  from  the  temperature  of  the  air 
through  which  the  rays  were  passing. 

The  supply  of  radiant  energy  is  very  much  in  excess  of 
the  amount  which  is  needed  for  the  internal  work.  Indeed 
its  absorption  by  the  leaves  would  be  a  source  of  consider- 
able danger  to  the  plant  were  it  not  for  the  cooling  effect 
of  transpiration,  which  we  have  seen  dissipates  98  per  cent, 
of  it  during  bright  sunshine.  No  doubt  this  dissipation  is 
one  of  the  chief  benefits  secured  by  transpiration. 

It  is  evident,  however,  that  in  the  general  economy  of 
the  plant  something  further  must  be  at  work  in  connection 
with  the  supply  of  energy.  The  absorption  of  these  external 
forms  must  take  place  at  the  exterior  of  the  plant,  while 
many  of  the  processes  of  expenditure  are  carried  out  in 
parts  which  are  more  or  less  deep-seated.  We  are  obliged 
to  turn  our  attention,  therefore,  in  this  connection  as  in 
that  of  the  construction  and  utilisation  of  food,  to  processes 
of  accumulation,  distribution,  and  economy. 

We  may  ask  ourselves  what  is  the  immediate  fate  of  the 
energy  absorbed.  It  enters  the  plant  in  what  is  known 
as  the  kinetic  form.  A  very  considerable  part  of  the  kinetic 
energy  of  the  sun's  rays,  we  have  already  seen,  is  devoted 
at  once  to  the  evaporation  of  the  water  of  transpiration, 
but  some  of  it  is  employed  by  the  chloroplasts  to  construct 
some  form  of  carbohydrate.  The  energy  so  applied  can 
be  again  set  free  by  the  decomposition  of  this  manufactured 
material.  If  the  latter  were  burned  its  combustion  would 


288  VEGETABLE  PHYSIOLOGY 

be  attended  by  the  evolution  of  a  certain  definite  amount 
of  heat.  This  heat  would,  represent  the  energy  that  had 
been  applied  to  the  construction  of  the  material  so  burned. 
Any  accumulation  of  material  in  the  body  of  the  plant 
represents,  therefore,  not  only  a  gain  of  weight  or  substance, 
but  a  storage  of  energy.  This  has  disappeared  from  observa- 
tion during  the  constructive  processes,  but  can  be  liberated 
again  during  their  decomposition  and  applied  to  other  pur- 
poses. Energy  which  has  thus  been  accumulated  and 
stored  is  known  as  potential  energy,  to  distinguish  it  from 
the  actual  or  'kinetic  energy  originally  absorbed.  The  forma- 
tion of  material  in  the  plant,  therefore,  involves  a  storage 
of  energy  in  the  potential  form,  and  wherever  such  material 
is  found  there  is  in  it  an  amount  of  energy  which  can  be 
liberated  with  a  view  to  utilisation  at  any  point  to  which 
the  material  has  been  transferred.  The  translocation  of 
material,  therefore,  involves  also  a  distribution  of  the 
energy  which,  originally  absorbed  as  the  kinetic  energy  of 
light  or  heat,  has  been  applied  to  constructive  processes,  and 
has  consequently  been  made  potential. 

Those  exceptional  plants  which  absorb  elaborated  food 
from  their  environment  have  a  source  of  energy  therein. 
This  food  is  a  store  of  potential  energy  which  is  absorbed 
as  such,  and  made  kinetic  subsequently.  The  salts  absorbed 
by  all  plants  from  the  soil  also  represent  a  certain,  though 
very  small,  amount  of  potential  energy. 

It  is  this  potential  energy  on  which  the  plant  depends 
for  the  various  processes  which  go  on  in  such  cells  as  are 
not  the  recipients  of  external  kinetic  energy.  Even  in  the 
cells  which  absorb  the  latter  a  certain  amount  of  potential 
energy  also  is  present,  which  represents  what  has  been 
stored  by  them  in  the  constructive  processes  they  carry  out, 
or  has  reached  them  in  the  shape  of  complex  materials 
formed  originally  in  other  cells. 

It  is  mainly  on  this  store  that  not  only  the  whole  organism, 
but  every  cell  depends  for  the  execution  of  its  vital  processes. 
Each  cell  is  a  seat  of  the  liberation  of  this  potential  energy, 


THE  ENERGY  OF  THE  PLANT  289 

or  its  conversion  into  the  kinetic  form,  during  the  decom- 
positions which  take  place  within  it. 

The  protoplasm  itself  contains  a  store  of  such  potent^ 
energy.  We  have  seen  that  it  can  only  be  constructed  at 
the  expense  of  food  supplied  to  it.  The  formation  of  the 
protoplasm  which  follows  the  supply  of  food  to  the  cell 
involves  work,  and  the  energy  so  used  is  partly  changed 
from  the  kinetic  to  the  potential  condition.  When  the 
protoplasm  undergoes  what  we  have  called  its  auto-decom- 
position, which  is  continually  taking  place,  a  certain  amount 
of  this  potential  energy  is  liberated  and  can  be  observed  and 
measured  in  various  ways.  When  destructive  metabolism 
is  active  we  have  already  noticed  that  there  is  usually  a 
rise  of  temperature,  as  in  the  processes  of  the  germination  of 
seeds.  A  certain  amount  of  the  liberated  potential  energy 
in  this  case  manifests  itself  in  the  form  of  heat.  A  vegetable 
cell  which  obtains  no  direct  radiant  energy  from  without 
can  consequently  obtain  the  energy  it  needs  from  within 
itself,  by  setting  up  decomposition  either  of  its  own  sub- 
stance or  of  certain  materials  which  have  been  accumulated 
within  it. 

The  supply  of  elaborated  material  to  a  cell  and  that  of 
available  potential  energy  within  it  are  not,  however, 
exactly  equivalent.  A  certain  part  of  the  transported 
material  is  devoted  to  the  maintenance  of  the  fabric  of  the 
cell.  The  protoplasm  in  a  growing  cell  is  permanently 
increased  ;  frequently  its  cell- wall  is  permanently  thickened. 
In  these  cases  the  whole  of  such  material  is  not  subjected 
to  subsequent  decomposition,  but  much  remains  unchanged 
during  the  plant's  life.  The  cell  is  consequently  never 
found  to  be  capable  of  giving  up  to  the  plant  of  which  it  is 
a  member  the  whole  of  the  potential  energy  which  reaches 
it.  If  we  consider  the  round  of  the  metabolic  changes 
which  take  place  in  such  a  cell,  we  find  that  energy  is 
absorbed  to  construct  its  substance,  and  that  as  the  latter 
undergoes  self-decomposition  energy  is  again  liberated. 
But  a  certain  part  of  what  is  supplied  to  it  is  permanently 

19 


290  VEGETABLE  PHYSIOLOGY 

retained  in  potential  form,  and  hence  every  cell  depends 
for  the  maintenance  of  its  energy  upon  a  constant  supply 
of  complex  material,  whose  subsequent  decompositions 
will  replace  the  amount  of  energy  which  has  been  utilised 
in  the  permanent  increase  of  its  substance. 

The  translocation  of  constructed  materials  which  we 
have  already  considered  must  be  regarded,  therefore,  not 
only  as  furnishing  the  materials  for  nutrition  and  growth, 
but  also  as  carrying  or  distributing  throughout  the  plant- 
body  the  kinetic  energy  absorbed  as  light  or  heat  by  the  cells 
to  which  these  forces  are  originally  supplied.  The  chloro- 
phyll apparatus  is  an  important  piece  of  mechanism  for 
the  accumulation  of  energy  which  is  subsequently  distri- 
buted and  utilised  wherever  need  arises  for  it.  This  is 
true  also  of  all  cells  which  have  the  power  of  absorbing 
kinetic  energy  in  any  form. 

The  absorption  and  fixation  of  energy  involved  in  the 
photosynthetic  processes  carried  out  by  the  chlorophyll 
apparatus  can  be  easily  observed,  and  the  immediate  fate 
of  such  energy  can  be  readily  determined.  The  accumu- 
lation of  the  energy  of  heat  is  not  so  easy  to  trace,  but 
there  is  no  doubt  that  it  proceeds  along  similar  lines.  Part 
of  it  can  travel  as  kinetic  energy,  as  heat  is  slowly  con- 
ducted along  the  tissues  of  the  plant,  but  ultimately  some, 
at  any  rate,  becomes  potential. 

We  see,  therefore,  that  wherever  any  substance  that 
has  been  manufactured  by  the  plant  is  stored  in  a  cell,  that 
cell  is  thereby  put  in  possession  of  a  certain  amount  of 
potential  energy  corresponding  to  the  quantity  of  such 
stored  material.  Even  the  living  substance  itself  may  be 
looked  upon  as  a  further  store  of  energy,  as  it  can  liberate 
it  by  its  own  decomposition. 

Each  cell  is  thus  supplied  through  the  general  activities 
of  the  whole  plant,  not  only  with  the  food  it  needs  for  its 
nutrition,  but  also  with  the  energy  required  for  carrying  out 
its  vital  processes.  The  ultimate  utilisation  of  the  stored 
energy  is  consequently  a  process  which  must  be  studied 


THE  ENERGY  OF  THE  PLANT  291 

by   a    close    scrutiny    of    the    internal    work    of   the    cell 
itself. 

The  transformation  of  potential  into  kinetic  energy  is 
associated  with  decomposition  just  as  the  converse  process 
is  bound  up  with  construction.  Destructive  metabolism 
in  the  cell  is  then  the  means  by  which  its  energy  is  made 
available.  We  have  seen  that  the  processes  of  this  kata- 
bolism  go  on  in  the  interior  of  each  cell.  Each  liberates 
at  least  as  much  energy  as  it  requires  for  the  maintenance 
of  its  life  and  the  discharge  of  its  particular  functions. 

The  processes  associated  with  the  utilisation  of  the 
stored  energy  are,  then,  chemical  decompositions  in  which 
various  constituents  of  the  cell  are  involved.  We  may 
divide  them  into  two  series,  in  the  first  of  which  the  proto- 
plasm itself  takes  part,  and  which  comprise  the  processes 
in  which  its  own  breaking  down  takes  place.  In  the  second 
series  it  effects  the  splitting  up  of  other  bodies  without  a 
necessary  disruption  of  its  own  molecules. 

The  first  of  these  two  series  involves  the  phenomena  of 
respiration,  to  which  we  must  now  turn  our  attention. 

Of  the  gaseous  interchanges  which  were  mentioned  in  a 
former  chapter  as  characteristic  of  living  protoplasts,  the 
most  widespread  is  that  which  is  marked  by  the  absorption 
of  oxygen.  With  the  exception  of  a  few  of  the  lowlier 
organisms,  all  of  which  are  members  of  the  group  of  Fungi, 
every  living  protoplast  must  be  constantly  absorbing 
this  gas  in  order  not  only  that  its  vital  activities  may  con- 
tinue to  be  discharged,  but  that  its  life  itself  may  be  main- 
tained. Withdrawal  of  oxygen  from  the  environment 
of  the  protoplast  is  after  a  longer  or  shorter  interva 
followed  by  its  death.  It  is  true  that  under  certain  con- 
ditions which  we  shall  discuss  in  a  subsequent  chapter 
the  interval  may  be  prolonged,  but  death  ultimately 
ensues. 

This  absorption  of  oxygen  is  in  most  cases  associated 
with  an  exhalation  of  carbon  dioxide,  which  is  generally 
given  off  in  a  volume  approximately  equal  to  that  of  the 

19  * 


292  VEGETABLE  PHYSIOLOGY 

oxygen  taken  in.     It  is  always  accompanied  or  followed 
by  the  formation  of  a  certain  amount  of  watery  vapour. 

The  universality  of  this  process  is  not  always  easy  to 
demonstrate.  It  can  be  ascertained  without  difficulty  in 
the  case  of  almost  all  animal  organisms,  and  of  such  of 
the  vegetable  ones  as  possess  no  chlorophyll.  In  the  case 
of  those  plants  which  are  green,  however,  there  is,  as  we 
have  seen  in  a  preceding  chapter,  a  converse  gaseous  inter- 
change occurring  so  long  as  the  green  parts  are  exposed 
to  sunlight,  carbon  dioxide  being  absorbed  and  decom- 
posed, and  an  equal  amount  of  oxygen  exhaled.  This 
interchange  is  usually  more  vigorous  than  the  first  one,  and 
the  latter  is  therefore  difficult  of  detection  under  conditions 
which  allow  both  to  take  place  simultaneously. 

The  absorption  of  oxygen  can  be  easily  observed  in  the 
case  of  a  large  fungus,  such  as  a  mushroom.  If  one  of  these 
plants  be  placed  in  a  closed  receiver  containing  air,  and 
left  there  for  several  hours,  at  the  conclusion  of  the  experi 
ment  the  mixture  of  gases  in  the  receiver  will  be  found  to 
be  almost  devoid  of  oxygen,  that  which  was  there  originally 
having  disappeared.  An  almost  equal  amount  of  carbon 
dioxide  will  be  found  to  have  replaced  it,  so  that  the  volume 
of  gas  in  the  receiver  will  be  practically  unaltered. 

It  is  possible  to  devise  an  experiment  which  will  show 
that  a  green  plant  has  the  same  absorbing  power.  If  the 
light  is  excluded  from  one  placed  in  a  similar  vessel,  no 
evolution  of  oxygen  will  take  place  from  it,  and  that  the 
oxygen  present  in  the  air  at  the  commencement  of  the 
observation  will  diminish  to  the  point  of  extinction  can 
be  made  evident,  just  as  in  the  case  of  the  mushroom. 

We  have  evidence,  however,  that  this  is  not  caused 
by  the  exclusion  of  the  light,  but  that  the  gaseous  inter- 
change in  question  proceeds  in  the  light  as  well  as  in  dark- 
ness. An  apparatus  which  was  originally  devised  by 
Garreau  can  be  easily  arranged  to  show  the  absorption 
of  oxygen,  even  when  a  green  plant  is  exposed  to  a  bright 
sunlight.  There  are  many  forms  of  it,  but  a  convenient 


KESPIEATION 


298 


one  consists  of  a  glass  vessel  which  can  be  closed  by  a  cork, 
through  which  a  bent  glass  tube  of  small  calibre  is  passed. 
The  tube  is  carried  over  and  made  to  dip  into  a  small  dish 
containing  mercury.  The  bottom  of  the  vessel  is  covered 
with  finely  broken  glass,  upon  which  is  poured  a  strong 
solution  of  caustic  potash.  Above  the  latter,  supported  by 
the  glass  so  as  not  to  be  in  contact  with  the  alkali,  is  placed 
the  plant  to  be  examined  (fig.  131).  Watercress  or  any 
other  herbaceous  plant  will  answer  very  well.  The  potash 
will  absorb  the  carbon  dioxide  of  the  atmosphere  originally 
admitted,  as  well  as  whatever  quantity  of  this  gas  is  given 
off  during  the  experiment.  As 
the  experiment  progresses  the  tem- 
perature must  be  kept  constant, 
when  the  mercury  will  be  found 
to  rise  slowly  and  gradually  in  the 
small  glass  tube,  indicating  a 
diminution  of  the  volume  of  the 
air  in  the  flask.  If  the  experi- 
ment is  continued  till  the  mercury 
ceases  to  rise  in  the  tube,  and 
the  gas  remaining  in  the  vessel 
is  measured  at  the  ordinary  at- 
mospheric pressure,  and  at  the 
temperature  at  which  the  experi- 
ment was  started,  it  will  be  found 
that  its  volume  has  been  diminished 

by  about  twenty  per  cent.,  and  that  what  is  left  consists  of 
nitrogen.  The  oxygen  will  have  been  completely  removed 
by  the  green  plant,  even  when  the  apparatus  is  left  ex- 
posed to  the  sunlight  during  the  daytime.  If  the  caustic 
potash  is  examined,  it  will  be  found  to  have  gained  con- 
siderably in  weight,  and  to  contain  a  quantity  of  carbonate 
of  potassium,  which  must  have  been  derived  from  the  plant 
during  the  experiment.  The  weight  of  this  will  enable  the 
volume  of  the  evolved  carbon  dioxide  to  be  ascertained. 
There  will  have  been  proceeding  during  the  experiment  an 


FIG.  131. — APPARATUS  TO  SHOW 
THE  ABSORPTION  OF  OXYGEN 
BY  A  GREEN  PLANT. 


294 


VEGETABLE  PHYSIOLOGY 


absorption  of  oxygen,  attended  as  before  by  an  exhalation 
of  carbon  dioxide,  the  latter  having  combined  with  the 
potash. 

The  evolution  of  carbon  dioxide  by  the  plant  can  be 
more  easily  demonstrated  by  the  use  of  the  apparatus 
shown  in  fig.  132.  The  jar  A  in  the  centre  contains  the 
plant  to  be  examined,  which  may  preferably  be  represented 
by  a  number  of  germinating  peas.  It  is  closed  by  a  cork, 
which  is  perforated  in  two  places.  Into  one  hole  a  tube  is 
inserted  which  passes  to  the  bottom  of  the  jar,  and  serves 
for  the  admission  of  air.  An  outlet  tube  passes  through 


FIG.  132. — APPARATUS  TO  SHOW  THE  EXHALATION  OF  CARBON  DIOXIDE  BY  GER- 
MINATING SEEDS.  THE  AIR  ENTERS  THROUGH  THE  TUBE  ON  THE  LEFT  ;  ITS 
CARBON  DIOXIDE  is  ABSORBED  BY  THE  POTASH  IN  F.  IT  PASSES  THROUGH  A, 

IN  WHICH  THE  SEEDS  ARE  PLACED,  AND  THE  CARBON  DlOXIDE  GENERATED  THERE 
IS  CARRIED  OVER  INTO  C,  WHERE  IT  IS  PRECIPITATED  BY  THE  BARYTA  WATER. 


the  other  hole  from  the  upper  part  of  the  jar,  and  leads  to 
another  jar,  c,  which  is  partially  filled  with  baryta  water. 
The  final  outlet  from  c  can  be  attached  to  an  aspirator  by 
which  a  stream  of  air  can  be  drawn  through  the  apparatus. 
Before  the  incoming  air  reaches  the  jar  A  it  is  made  to  pass 
through  another  jar,  F,  containing  a  solution  of  caustic 
potash  which  frees  it  from  all  traces  of  carbon  dioxide. 
To  ascertain  that  this  is  secured,  it  passes  next  through  a  jar 
B  which  contains  baryta  water.  A  stream  of  air  is  then 
passed  slowly  and  continuously  through  the  whole  apparatus, 
and  as  it  bubbles  through  the  baryta  water  in  o  it  causes 
the  formation  of  a  white  precipitate,  which  analysis  shows 


KESPIKATION  295 

to  be  barium  carbonate.  The  formation  of  this  body 
proves  the  exhalation  of  carbon  dioxide  by  the  seeds, 
as  the  entering  air  contains  none.  By  separating  and 
weighing  the  barium  carbonate  precipitated  in  c,  the  amount 
of  the  gas  evolved  in  a  definite  time  may  easily  be  ascer- 
tained. 

If  the  tubes  between  B  and  A  and  A  and  o  be  cut  and 
joined  by  a  narrow  india-rubber  pipe  which  can  be  com- 
pressed by  a  metal  clip,  the  jar  A  can  be  isolated  and  the 
carbon  dioxide  allowed  to  accumulate  in  it. 

These  two  processes,  the  absorption  of  oxygen  and  the 
exhalation  of  carbon  dioxide,  are  characteristic  of  what  is 
known  as  respiration.  As  already  stated,  it  is  a  normal 
process  of  the  life  of  almost  all  protoplasm,  and  is  con- 
tinually going  on  so  long  as  life  lasts,  although  it  is  not 
easily  observed  while  the  converse  process,  the  absorption 
and  decomposition  of  carbon  dioxide,  is  proceeding,  accom- 
panied by  the  exhalation  of  oxygen.  It  is  frequently  said 
that  during  daylight  the  process  of  the  respiration  of  a 
green  plant  is  masked  by  that  of  carbon  dioxide  decom- 
position. To  put  this  statement  into  somewhat  different 
terms,  the  carbon  dioxide  which  is  liberated  in  the  course 
of  respiration  by  the  green  plant,  and  which  is  in  com- 
paratively small  amount,  is  re-absorbed  by  the  green  parts 
of  the  cells,  and  undergoes  the  same  decomposition  as  that 
which  is  brought  to  the  plant  by  the  surrounding  air.  It 
thus  escapes  observation  unless  special  means,  such  as 
those  detailed,  are  adopted  to  bring  it  into  evidence. 

The  respiratory  processes  are  easily  observed  in  the 
case  of  all  plants,  and  parts  of  plants,  that  are  not  green, 
as  there  are  in  such  cases  no  gaseous  interchanges  that 
would  interfere  with  their  manifestation. 

If  a  plant  be  carefully  weighed  at  the  commencement 
and  at  the  end  of  such  an  experiment  as  has  been  described, 
it  will  be  found  to  have  lost  weight  during  its  stay  in  the 
receiver,  so  that  respiration  is  associated  with  a  loss  of 
weight  to  the  plant.  This  may  readily  be  inferred  from 


296  VEGETABLE  PHYSIOLOGY 

the  fact  that  the  oxygen  absorbed  and  the  carbon  dioxide 
exhaled  are  approximately  equal  in  volume,  carbon  dioxide 
being  perceptibly  heavier  than  oxygen.  Besides  the 
carbon  dioxide,  however,  there  is  always  also  a  certain 
exhalation  of  watery  vapour,  which  takes  place  quite 
independently  of  any  supply  from  the  root  or  the  cut  end 
of  the  stem.  The  nature  of  the  metabolism,  or  the  vital 
processes,  is  such  that  the  living  substance  gives  off  both 
water  and  carbon  dioxide,  while  it  coincidently  absorbs 
oxygen.  This  is  quite  independent  of  any  constructive 
processes,  for  it  can  be  observed  when  no  nutritive  material 
of  any  kind  is  supplied  to  the  plant. 

Though  respiration  is  constantly  proceeding  wherever 
living  substance  is  found,  the  activity  of  the  process  is 
by  no  means  uniform.  With  care  it  can  be  detected  in 
such  quiescent  parts  of  plants  as  resting  seeds,  or  buds 
during  their  winter  suspension  of  development,  but  in 
such  cases  the  gaseous  interchange  is  reduced  to  a  minimum. 
In  growing  shoots  or  germinating  seeds  in  which  vital 
processes  such  as  the  growth  of  protoplasm  are  going  on 
rapidly,  and  life  is  very  active,  it  reaches  a  maximum. 
In  ordinary  adult  leaves  and  branches  the  activity  of 
respiration  is  intermediate  between  the  other  two  condi- 
tions. It  is  more  intense,  again,  in  the  floral  organs  during 
the  time  of  their  maturation.  We  may  say  in  general 
terms,  wherever  protoplasm  is  abundant,  and  the  chemical 
processes  connected  with  the  manifestation  of  its  life  are 
going  on  most  vigorously,  there  respiration  is  most  active. 
It  is  connected  especially  with  the  vital  processes,  and  is 
not  associated  directly  with  the  presence  of  food  materials. 
A  proof  of  this  is  afforded  by  an  estimation  of  the  activity 
of  respiration  in  seedlings,  which,  in  the  case  of  wheat,  has 
been  found  to  increase  steadily  for  about  a  fortnight,  and 
then  to  decline.  Further  evidence  is  afforded  by  the  fact 
that  if  seeds  are  thoroughly  dried  they  do  not  respire.  In 
this  condition  the  protoplasm  is  completely  quiescent,  so 
far  as  we  can  ascertain.  If,  however,  only  a  little  water  is 


BESPIRATION  297 

supplied  to  them,  which,  as  we  have  seen  in  an  earlier  chapter, 
is  a  condition  necessary  to  set  up  changes  in  the  protoplasm, 
respiration  commences,  and  increases  as  the  proportion  of. 
water  present  rises  up  to  a  certain  limit. 

When  the  respiratory  processes  are  carefully  measured 
and  compared  with  the  weight  of  the  organism,  it  is  found 
that  under  appropriate  conditions  they  are  more  intense  in 
plants  than  even  in  warm-blooded  animals.  The  respiratory 
activity  is  as  great  in  many  seedlings  as  it  is  in  the  human 
body,  provided  that  both  are  maintained  at  the  same  tem- 
perature. There  is,  however,  a  very  great  variability  in  this 
respect,  and  the  maximum  activity  is  never  maintained  very 
long  in  any  particular  plant.  As  maturity  succeeds  to 
development  its  amount  falls  materially,  being  marked  at 
or  near  the  original  rate  only  in  the  regions  of  the  active 
meris  terns. 

All  seedlings,  again,  are  not  alike  in  the  vigour  with 
which  they  carry  on  their  respiratory  processes. 

We  may  pass  on  to  inquire  what  is  the  relation  between 
the  absorption  of  oxygen  and  the  formation  and  elimination 
of  carbon  dioxide  and  water.  It  is  conceivable  that  the 
oxygen  may  unite  in  the  plant  with  carbon  and  with  hydro- 
gen to  produce  at  once  the  exhaled  compounds.  A  study 
of  the  living  organism  at  work,  however,  soon  shows  us 
that  the  process  is  not  of  this  simple  nature.  We  have 
said,  in  the  course  of  what  has  already  been  advanced,  that 
the  amount  of  the  carbon  dioxide  exhaled  and  that  of  the 
oxygen  absorbed  are  approximately  equal.  This,  however, 
is  only  true  within  certain  limits  ;  if  each  is  measured 
accurately,  they  are  not  found  to  show  an  exact  correspon- 
dence. The  ratio  C03 :  Oa  is  usually  spoken  of  as  the  respira- 
tory quotient.  When  the  two  processes  are  equal  the  value 
of  the  respiratory  quotient  is  unity ;  when  the  carbon 
dioxide  is  in  excess  it  is  greater,  and  when  the  oxygen 
is  in  largest  amount  it  is  less,  than  unity.  The  respiratory 
quotient  has  been  found  to  vary  to  a  greater  or  less  extent 
in  different  plants,  and  in  the  same  plant  under  different 


298  VEGETABLE  PHYSIOLOGY 

conditions.  If  its  value  is  determined  in  the  case  of  ger- 
minating seeds,  these  differences  are  soon  evident.  With 
starchy  seeds  the  quotient  is  unity  ;  with  oily  seeds  it  is 
much  lower.  That  is,  in  the  former  case  the  seeds  absorb 
a  volume  of  oxygen  equal  to  that  of  the  carbon  dioxide 
they  exhale  ;  in  the  latter  case  they  take  up  more. 

Various  observers  have  shown  that  in  certain  cases  suc- 
culent leaves,  such  as  those  of  the  Agave  or  of  particular 
plants  belonging  to  the  Saxifragacece  and  the  Crassulacece, 
or  again  the  phylloclades  of  Opuntia,  one  of  the  Cactacece, 
are  capable  of  absorbing  oxygen  without  the  simultaneous 
exhalation  of  carbon  dioxide.  Nor  is  the  oxygen  absorbed 
in  these  cases  any  more  than  it  is  in  others  without  entering 
into  some  form  of  chemical  combination,  for  it  cannot  in 
any  case  be  extracted  by  the  air-pump.  The  latter  also 
fails  to  extract  any  carbon  dioxide  from  the  plants.  The 
oxygen  enters  the  plant,  and  is  in  some  way  fixed  or  com- 
bined ;  the  other  process  which  usually  accompanies  this 
absorption  does  not  take  place,  the  carbon  dioxide  not  only 
not  being  exhaled,  but  apparently  not  even  formed. 

Conversely,  carbon  dioxide  may  be  given  off  from  a 
plant  without  any  simultaneous  or  even  antecedent  absorp- 
tion of  oxygen.  When  a  seed  is  made  to  germinate  in  a 
vacuum  over  a  column  of  mercury,  carbon  dioxide  is  found  to 
be  liberated.  Eipe  fruits  have  been  found  to  give  off  this 
gas  in  an  atmosphere  quite  devoid  of  oxygen.  Too  much 
stress  must  not,  however,  be  laid  upon  these  latter  observa- 
tions, as  we  have  certain  evidence  which  points  to  a  different 
mode  of  formation  of  the  carbon  dioxide  in  the  presence  and 
in  the  absence  of  oxygen  respectively. 

Again,  it  is  found  that  the  respiratory  quotient  varies 
according  to  the  temperature  at  which  the  observations  are 
made.  This  is  explained  by  the  fact  that  both  the  absorption 
of  oxygen  and  the  exhalation  of  carbon  dioxide  vary  with 
differences  of  temperature,  but  they  vary  in  different 
degrees.  Evidently  the  two  processes  are  not  directly 
dependent  upon  each  other. 


BESPIKATION  299 

In  making  the  estimation  of  the  respiratory  interchanges 
we  are  apt  to  lose  sight  of  a  fact  to  which  attention  has 
already  been  called,  viz.  that  carbon  dioxide  is  not  the  only 
respiratory  exhalation.  The  watery  vapour  which  accom- 
panies it  must  also  be  accounted  for.  On  the  hypothesis 
of  the  direct  oxidation  of  carbon  and  hydrogen,  if  the  volume 
of  carbon  dioxide  is  equivalent  to  that  of  the  oxygen,  there 
cannot  have  been  the  absorption  of  sufficient  of  the  latter  to 
unite  with  hydrogen  to  form  the  water.  Even  when  the 
respiratory  quotient  is  less  than  unity,  the  same  considera- 
tion has  a  certain  value.  The  idea  of  such  direct  oxidation 
cannot,  therefore,  be  accepted. 

It  is  evident  from  the  foregoing  considerations  that  the 
vital  activity  of  the  protoplasts  is  somehow  associated 
with  the  two  factors  in  the  gaseous  interchange.  In  the 
absence  of  oxygen  this  vital  activity  gradually  ceases,  the 
living  substance  being  in  fact  slowly  stifled  or  asphyxiated. 
During  its  life  one  of  the  manifestations  of  its  vitality  is 
the  formation  and  exhalation  of  two  fairly  simple  com- 
pounds, carbon  dioxide  and  water.  To  ascertain  what  is 
the  true  relation  of  the  two  processes,  it  is  necessary  to 
look  closely  at  the  nature  of  the  chemical  changes  going 
on  in  the  protoplasm  itself,  or  what  is  usually  spoken  of  as 
its  metabolism. 

Eespiration  in  the  strict  sense  is  therefore  a  process 
going  on  in  the  living  substance  itself.  The  gaseous  inter- 
change observed  is  the  expression  of  the  beginning  and  the 
end  of  a  series  of  complex  changes  in  which  the  molecules 
of  the  living  substance  are  involved.  The  details  of  the 
absorption  of  the  oxygen  of  the  plant  from  its  environment, 
and  its  presentation  to  the  protoplasm,  together  with  those 
of  the  ultimate  exhalation  of  the  carbon  dioxide  and  water 
from  the  plant-body,  should  be  regarded  rather  as  belonging 
to  the  mechanism  of  respiration  than  to  respiration  itself, 
which  is  a  function  of  the  living  substance  only.  The  former 
corresponds  to  the  entry  of  the  oxygen  into  the  lungs  of 
an  air-breathing  mammal,  and  its  transport  to  the  tissues, 


300  VEGETABLE  PHYSIOLOGY 

together  with  the  return  of  the  carbon  dioxide  and  water 
therefrom ;  the  latter  is  strictly  comparable  to  the  changes 
taking  place  in  those  tissues  after  the  entry  of  the  oxygen 
into  them. 

The  variation  of  the  respiratory  quotient  which  we 
noticed  in  starchy  and  oily  seeds  respectively  points  to  a 
varied  metabolism,  according  to  the  nature  of  the  food 
supplied  to  the  living  substance. 

We  see,  then,  that  the  two  processes  are  not  immediately 
connected  in  the  sense  of  the  carbon  dioxide  and  water 
coming  at  once  from  the  direct  oxidation  of  carbon  and 
hydrogen,  but  that  they  are  ultimately  associated  there  can 
be  no  doubt,  though  they  are  separated  in  time  by  a  series 
of  chemical  changes  taking  place  in  the  living  substance. 
This  ultimate  association  is  shown  by  the  fact  that,  if 
the  access  of  oxygen  to  a  plant  is  prevented,  after  a  longer 
or  shorter  period  the  exhalation  of  carbon  dioxide  ceases. 

To  get  a  true  view  of  the  nature  of  the  process  of  respira- 
tion we  must  therefore  turn  our  attention  to  the  metabolic 
changes  which  are  taking  place  normally  in  the  living  sub- 
stance. From  the  instability  which  we  have  noticed  in 
the  protoplasmic  material,  we  can  infer  that  its  own  mole- 
cules are  in  a  constant  state  of  decomposition  and  recon- 
struction, new  material  being  incorporated  and  certain 
other  substances  cast  off.  Besides  these,  we  are  probably 
not  wrong  in  concluding  that  other  changes  also  take  place 
in  the  various  substances  which  are  contained  in  it,  into 
which  its  own  molecules  do  not  enter.  Processes  of  slow 
oxidation  and  gradual  reduction  are  taking  place  there 
continually,  excited,  however,  in  all  probability  by  the 
changes  in  the  protoplasm  itself.  We  shall  discuss  these 
later,  but  for  the  present  we  may  say  that  they  are  by  no 
means  simple,  and  the  direct  oxidation  of  either  carbon 
or  hydrogen  has  probably  no  place  among  them.  An 
instance  of  them  may  be  seen  in  the  oxidation  of  alcohol 
in  the  cells  of  Mycodermi  aceti,  a  fungus  which  converts 
alcohol  into  acetic  acid.  This  process,  into  which  the 


KESPIKATION  301 

molecule  of  protoplasm  apparently  does  not  enter,  can 
only  go  on  in  the  living  cell.  Other  similar  instances  could 
be  quoted. 

The  probable  course  of  events  in  respiration  is  that  the 
oxygen  in  some  way  unites  with  the  protoplasm,  rendering 
it  unstable,  and  initiating  a  series  of  decompositions  which 
result  in  the  successive  formation  of  many  bodies  of  less 
complex  composition,  each  successive  decomposition  pro- 
ducing simpler  ones,  till  finally  carbon  dioxide  and  water  are 
formed.  Simultaneously,  reconstruction  of  the  protoplasm 
goes  on,  many  of  these  residues,  instead  of  being  at  once 
decomposed,  being  in  whole  or  in  great  part,  together  with 
new  material  supplied  to  it  in  the  shape  of  food,  built  up 
again  into  its  substance,  and  again  broken  down  in  further 
decompositions.  If  the  temperature  is  low,  the  breaking 
down  of  the  protoplasm  proceeds  but  slowly,  and  recon- 
struction is  rapid,  so  that  under  these  conditions  the  quantity 
of  oxygen  absorbed  or  fixed  as  intramolecular  oxygen  by 
the  protoplasm  is  greater  than  the  quantity  of  carbon  dioxide 
formed  by  its  decomposition.  At  a  higher  temperature 
decomposition  is  much  more  easily  carried  on,  and  its 
products  are  more  numerous  and  simpler.  The  decom- 
position and  recomposition  go  on  side  by  side,  simpler  bodies 
being  gradually  produced,  either  by  their  splitting  from  the 
protoplasm  directly,  or  by  their  being  formed  at  the  expense 
of  the  more  complex  decomposition-products  during  processes 
of  slow  oxidation  in  the  substance  of  the  protoplasm,  till 
finally  a  certain  production  of  carbon  dioxide  and  water  is 
arrived  at.  So  long  as  the  protoplasm  remains  alive  the 
amount  of  these  is  relatively  small,  reconstruction  con- 
tinually taking  place.  When,  however,  the  protoplasm 
dies,  simple  bodies,  such  as  carbon  dioxide,  water,  and 
possibly  ammonia,  in  addition,  are  produced  abundantly 
from  the  decomposition  which  attends  its  death. 

If  the  auto-decomposition  of  protoplasm  during  life 
involved  such  a  splitting-up  as  would  lead  to  the  formation 
of  nothing  but  these,  nearly  all  the  potential  energy  of  the 


302  VEGETABLE  PHYSIOLOGY 

cell  would  be  liberated.  We  have  seen,  however,  that  this 
is  not  the  case,  but  that  a  good  deal  of  the  energy  set  free 
is  employed  in  the  reconstruction  of  the  protoplasm  from 
these  products  and  the  new  food  supplied.  As,  however, 
the  final  result  is  the  formation  of  a  certain  quantity  of  the 
simpler  bodies  mentioned;  there  is  always  a  balance  of 
energy  set  free. 

The  carbon  dioxide  is  thus  the  final  term  in  a  series 
of  decompositions,  of  which  the  living  substance  is  the  seat 
and  into  which  it  may  actually  enter,  the  decompositions 
themselves  being  promoted  by  the  access  of  oxygen.  In 
some  cases,  such  as  those  of  the  succulent  leaves  of  the 
Crassulacece  and  the  tissues  of  the  Cactus  already  alluded 
to,  this  final  term  is  not  reached,  no  carbon  dioxide  being 
formed.  We  have  no  reason  to  think  that  in  these  cases 
a  fundamentally  different  series  of  changes  is  set  up.  De 
Saussure  found  that  a  piece  of  stem  of  Opuntia  absorbed 
a  quantity  of  oxygen,  which  could  not  be  extracted  from 
it  by  the  air-pump.  The  fate  of  this  oxygen  must  have 
been  similar  to  that  which  is  absorbed  by  other  plants  ;  it 
must  have  entered  into  some  form  of  combination,  probably 
with  the  living  substance.  The  resulting  decompositions, 
though  taking  at  first  the  same  course  as  in  other  cases, 
did  not  go  so  far.  Instead  of  the  liberation  of  carbon 
dioxide,  there  was  found  a  considerable  increase  in  the 
amount  of  certain  organic  acids,  chiefly  malic  and  oxalic 
acids,  which  remained  in  the  cells,  and  which  probably 
represented  the  ultimate  products  of  the  decompositions. 

Though  respiration  is  always  proceeding  wherever 
there  is  living  protoplasm,  the  activity  of  the  process 
is  modified  by  different  physical  conditions.  Of  these, 
temperature  is  one  of  the  most  important.  There  is  a 
lower  limit,  beyond  which  it  appears  to  be  suspended, 
though  life  is  not  destroyed.  This  limit  varies  in  different 
plants,  but  is  generally  one  or  two  degrees  below  the  freezing 
point  of  water.  In  a  few  cases,  such  as  Conifers  and  Lichens, 
it  may  even  be  -10°  C.,  but  this  is  rare.  As  the  tempera- 


CALIFORNIA    COLLEGE 

of  PHARMACY  . 

KESPIKATION  308 

ture  rises  from  this  minimum  point,  the  activity  of  respira- 
tion increases  up  to  a  certain  optimum  point,  which  is  usually 
not  well  denned,  and  which  varies  considerably  in  different 
plants.  If  the  temperature  is  raised  only  a  little  higher 
than  this,  the  living  substance  is  rapidly  injured,  and  its 
respiration  is  checked.  Variations  in  temperature  do  not 
affect  equally  the  absorption  of  oxygen  and  the  exhalation  of 
carbon  dioxide.  At  low  temperatures  the  latter  is  smaller 
than  the  former  ;  at  high  ones  the  reverse  is  the  case. 

The  effect  of  light  upon  respiration  is  not  very  marked 
and  is  probably  indirect.  Plants  which  grow  in  shady  spots 
usually  manifest  less  respiratory  activity  than  similar  ones 
growing  in  bright  sunlight,  but  this  may  be  the  result  of 
the  difference  in  the  amount  of  nutritive  material  they 
obtain,  which  is  incident  to  the  difference  in  their  situation. 
As  we  shall  see  in  a  subsequent  chapter,  light  has  a  very 
marked  influence  on  the  metabolic  processes,  and  its 
indirect  effects  may  be  very  far-reaching. 
(•  Eespiration  is  considerably  affected  by  variations  in  the 
amount  of  oxygen  which  the  environment  of  the  plant 
contains.  The  protoplasts  can  absorb  even  the  least  traces 
of  the  gas  which  reach  them,  but  a  certain  amount  is 
necessary  for  them  to  maintain  a  healthy  condition.  Great 
variations  are  not  usually  met  with,  but  on  the  summits  of 
high  mountains  there  is  much  less  available  for  them  than 
at  the  sea-level.  If  the  amount  of  oxygen  in  the  atmosphere 
from  any  cause  falls  below  about  5  per  cent.,  respiration  is 
seriously  impeded.  Similarly  plants  cannot  thrive  in  the 
presence  of  too  great  an  amount.  When  the  pressure  of  the 
gas  attains  the  amount  of  twenty  to  thirty  atmospheres, 
respiration  becomes  very  difficult  and  after  a  short  time 
ceases,  and  death  ensues. 

The  process  of  respiration  is  also  affected  to  a  consider- 
able extent  by  the  nature  of  the  substances  which  serve  as 
nutritive  material  for  the  reconstruction  of  the  protoplasm. 
It  has  already  been  pointed  out  that  seeds  containing  oil 
absorb  more  oxygen  during  germination  than  those  which 


304  VEGETABLE  PHYSIOLOGY 

contain  principally  starch.  Fungi  which  are  fed  with  car- 
bon-compounds that  contain  relatively  little  oxygen  give 
off  relatively  less  carbon  dioxide  than  others  which  are 
supplied  with  food  containing  a  large  percentage  of  this 
constituent.  Organs  which  contain  much  protein  matter 
respire  more  copiously  than  others  which  contain  but  little. 
The  nature  of  the  inorganic  salts  absorbed  also  influences 
the  process  to  a  certain  extent,  though  probably  these  only 
-act  indirectly. 

Eespiration  is  thus  to  be  looked  upon  as  a  process  very 
largely  connected  with  the  utilisation  of  the  store  of  energy 
which  each  cell  possesses,  and  to  be  perhaps  primarily 
concerned  in  the  transformation  of  that  energy  from  the 
potential  to  the  kinetic  form.  The  oxygen  appears  to  be 
necessary  mainly  for  the  purpose  of  exciting  those  decom- 
positions of  the  protoplasm  which  are  so  dependent  upon 
its  instability.  It  is  not,  however,  certain  that  this  is  the 
only  part  it  plays.  It  is  possible  that  some  of  the  products 
of  the  protoplasmic  disruption  are  oxidisable  substances, 
and  that  to  a  certain  extent  a  direct  oxidation  of  them  takes 
place.  There  is  undoubtedly  some  evidence  pointing  in 
that  direction. 

We  have,  besides  the  true  respiratory  processes,  a  second 
series  of  chemical  decompositions  going  on  in  plants,  pro- 
bably in  many  cases  closely  allied  to  those  of  the  first,  if 
not  inseparable  from  them,  but  differing  in  that  the  auto- 
decomposition  of  protoplasm  is  not  necessarily  involved. 
We  have  seen  already  that  many  processes  of  oxidation  and 
reduction  are  probably  always  taking  place  among  the  sub- 
stances which  are  in  solution  in  the  water  with  which  the 
cytoplasm  is  saturated.  Besides  these,  other  changes  take 
place  in  which  no  oxidation  is  involved,  and  this  whether 
oxygen  is  present  or  not.  If  the  access  of  oxygen  to  a 
protoplast  is  interfered  with,  its  normal  respiration  soon 
ceases,  but  very  frequently  other  changes  supervene,  involv- 
ing decompositions  of  a  different  character,  which  yield,  at 
any  rate  for  a  time,  the  energy  required  for  life. 


KESP1KATION  305 

Turning  from  the  question  of  respiration  to  study  other 
changes  which  subserve  a  similar  purpose  with  regard  to 
the  local  supply  of  energy,  we  may  first  examine  such  pro- 
cesses as  are  oxidative.  In  them  all  we  cannot  fail  to  mark 
the  activity  of  the  protoplasm  in  carrying  them  out.  The 
living  substance  does  not,  however,  act  as  a  general  oxidising 
agent,  but  different  protoplasts  possess  specific  powers. 
Certain  micro-organisms  can  cause  the  oxidation  of  ammonia 
and  the  consequent  formation  of  a  nitrite ;  others  can 
convert  the  nitrite  into  a  nitrate,  but  neither  can  do  the 
work  of  the  other.  Others  have  not  such  limited  powers  ; 
a  certain  bacterium  can  cause  the  oxidation  of  alcohol 
to  acetic  acid,  and  after  the  exhaustion  of  what  alcohol 
may  be  present,  can  further  oxidise  the  acetic  acid  to  carbon 
dioxide  and  water.  The  exact  way  in  which  the  protoplasm 
acts  as  a  carrier  of  the  oxygen  without  apparently  undergoing 
decomposition  is  very  obscure.  It  may  perhaps  combine 
with  the  oxygen  and  pass  it  on  to  these  oxidisable  substances, 
acting  as  a  carrier  only. 

It  has  recently  been  found  that  besides  exerting  a  direct 
oxidative  power,  protoplasm  can  secrete  an  enzyme,  or 
perhaps  a  variety  of  enzymes,  each  with  a  special  peculiarity, 
through  whose  instrumentality  the  oxidation  is  effected. 
These  enzymes  have  been  termed  oxidases,  and  they  are 
probably  widespread  in  the  vegetable  kingdom.  A  dis- 
cussion of  their  peculiarities  would  be  beyond  the  scope  of 
this  volume,  but  we  may  call  attention  to  their  general 
features. 

The  first  one  discovered  is  known  as  laccase ;  it  has  a 
very  wide  distribution,  occurring  in  the  roots,  stems,  and 
leaves  of  various  plants,  and  in  a  very  large  number  of 
fungi.  It  appears  to  oxidise  various  constituents  of  plants, 
but  particularly  the  colouring  matters.  Another,  known 
as  tyrosinase,  occurs  in  other  fungi,  and  oxidises  chiefly 
tyrosin.  Others  oxidise  various  colouring  matters,  together 
with  tannin.  Most  of  them  require  the  presence  of  a 
peroxide  in  the  cells  to  enable  them  to  act. 

20 


306  VEGETABLE  PHYSIOLOGY 

Many  very  complex  disturbances  set  in  when  a  normally 
respiring  plant  is  cut  off  from  a  supply  of  oxygen.  Death 
does  not  immediately  supervene,  as  might  almost  be  expected. 
Instead,  the  partial  asphyxiation  or  suffocation  stimulates 
the  protoplasm  to  set  up  a  new,  and  perhaps  supplementary, 
series  of  decompositions,  resulting  in  the  liberation  of 
energy,  as  do  those  of  the  respiratory  process.  We  have 
already  noticed  that  under  such  circumstances  the  exhaling 
stream  of  carbon  dioxide  can  still  be  observed.  This  led 
originally  to  the  view  that  the  protoplasm  excited  these 
decompositions  of  some  complex  substance  in  the  cell  to 
obtain  oxygen  from  it,  which  should  replace  the  oxygen 
whose  access  had  been  stopped.  The  ultimate  changes 
were  accordingly  held  to  go  on  with  but  slight  interruption, 
but  the  source  of  the  oxygen  taking  part  in  them  was 
different.  On  this  account  the  process  was  termed  intra- 
molecular respiration.  The  term  is  rather  an  unfortunate 
one,  for,  as  we  have  seen,  the  study  of  the  ordinary  respira- 
tory processes  has  shown  that  the  molecule  of  the  living 
substance  is  the  seat  of  the  changes  they  involve,  and 
hence  that  all  respiration  is  intramolecular.  Moreover,  if  the 
object  of  the  decomposition  is  to  provide  oxygen  to  replace 
that  which  has  been  cut  off,  these  transformations  precede 
the  actual  respiration,  which  must  then  be  set  up  as  soon 
as  the  oxygen  is  liberated  as  suggested.  Many  botanists 
now  prefer  to  speak  of  decompositions  taking  place  in  the 
absence  of  a  supply  of  free  oxygen  as  anaerobic  respiration. 
They  thus  include  as  respiratory  changes  all  the  decomposi- 
tions primarily  intended  to  liberate  energy,  and  divide  them 
into  those  which  are  aerobic  or  dependent  on  oxygen,  and 
those  which  are  anaerobic.  The  latter  need  not  involve 
the  co-operation  of  oxygen  in  the  disruption  of  the  molecule. 
The  object  sought  is  energy  and  not  oxygen. 

It  is  uncertain  how  far  the  auto-decomposition  of  proto- 
plasm is  concerned  in  these  anaerobic  respiratory  processes. 
Probably  not  to  any  great  extent ;  it  is  more  likely  that  it 
secures  the  decomposition  of  other  substances  without  being 


307 

"If          <Q*.    "<<^s 

itself  materially  used 
the  more   economical, 
much    energy   in   recons 
however,  be  regarded  as  fii 

The  substance  which  seem] 
purpose  is  sugar.  Under  the  coi 
decomposed  or  broken  up  entire! 
being  carbon  dioxide  and  alcohol, 
formation  which  was  for  so  long  a  time  assocl 
with  the  word  fermentation,  was  first  observed  in  connect 
with  the  life  of  the  yeast-plant.  It  has,  however,  since 
been  ascertained  to  be  much  more  widespread,  and  to  be 
indeed  the  most  common  of  the  anaerobic  respiratory 
processes.  In  cases  where  the  metabolic  activities  are  very 
great,  as  in  germinating  peas,  we  find  this  process  supple- 
ments the  ordinary  respiration,  for  alcohol  can  be  detected 
in  their  cells  in  small  quantities.  The  same  thing  has  been 
noticed  in  the  leaves  of  the  vine.  We  must  suppose  here 
that  the  amount  of  oxygen  absorbed  is  insufficient  for  their 
requirements,  and  that  partial  asphyxiation  results.  The 
swelling  of  the  germinating  seed  partly  occludes  its  inter- 
cellular spaces  and  so  hinders  the  access  of  oxygen  to  the 
cells  of  the  interior. 

It  was  for  a  long  time  held  that  alcoholic  fermentation 
was  conducted  exclusively  by  the  activity  of  the  proto- 
plasm of  the  cells  in  which  it  was  observed.  It  has  been 
ascertained,  however,  that  it  may  also  be  caused  by  the 
action  of  an  enzyme  zymase,  which  is  secreted  under  con- 
ditions of  incipient  asphyxiation  by  many  cells,  and  which 
is  formed  in  the  yeast-plant  even  in  the  absence  of  such 
stimulus. 

Though  the  term  *  fermentation  '  was  originally  applied 
and  confined  to  the  formation  of  alcohol,  it  is  now  usual  to 
extend  it  far  more  widely.  Many  other  processes  of  similar 
nature  have  been  discovered,  nearly  all  of  which  at  first 
were  found  to  be  carried  out  through  the  agency  of  microbes 
or  higher  fungi.  Hence  the  meaning  of  the  term  was 

20* 


308  VEGETABLE  PHYSIOLOGY 

extended  to  include  them,  and  the  organisms  themselves 
were  called  ferments.    As,  however,  these  processes  have 
come  to  be  recognised  as  normal  in  many  of  the  higher 
plants,  and  to  be  carried  out  in  them  by  the  protoplasm  of 
particular  cells,  this  peculiarity  is  seen  not  to  be  special  to 
the  microbes  and  the  fungi.     The  idea  was  soon  transferred 
to  the  protoplasm  in  general,  and  this  property  of  setting 
up  anaerobic  decomposition  became  known  as  its  fermenta- 
tive power.     The  very  similar  processes  set  up  through  the 
enzymes  which  we  have  discussed  in  connection  with  diges- 
tion show  us  another  manifestation  of  the  same  fermentative 
power.     All  these  processes  can  therefore  be  classed  under 
the   one  term  fermentation.     We   have   seen  that  all   the 
katabolic  changes  in  which  the  self-decomposition  of  the 
protoplasm  is  not  directly  involved  may  be  carried  out 
either  by  the  intervention  of  the  living  substance  itself 
or  an  enzyme  secreted  by  it.     The  oxidation  of  various 
matters  is  in  some  cases  confined  to  the  substance  of  the 
protoplasm  itself,  and  is  in  others  carried  out  in  its  vacuoles 
by  an  oxidase  ;  alcoholic  fermentation  is  in  some  cells  a 
matter  initiated  and  carried  on  by  their  protoplasm,  and  in 
others  is  due  to  the  enzyme  secreted  by  them.     The  digestive 
changes  can  similarly  be  conducted  by  enzymes  or  by  the 
living  substance  without  their  intervention. 

We  must  not,  however,  include  all  digestive  fermenta- 
tive changes  among  anaerobic  respiratory  phenomena,  if 
such  inclusion  involves  the  acceptance  of  the  view  that 
this  is  their  primary  purpose.  Though  they  do  effect 
the  conversion  of  potential  into  kinetic  energy,  this  is 
wholly  subsidiary  to  their  function  in  connection  with  the 
nutrition  of  the  plant.  We  have  seen  that  in  the  processes 
of  germination  the  energy  they  liberate  is  so  far  in  excess 
of  the  requirements  of  the  cells  that  a  large  amount  escapes 
in  the  form  of  heat.  For  them  to  work,  indeed,  there  must 
be  an  initial  supply  of  energy,  which  is  probably  supplied  to 
them  in  a  similar  form,  for  at  0°  C.  they  are  incapable  of 
effecting  any  decompositions. 


FEKMENTATION  309 

We  must  not  suppose  that  anaerobic  respiration  is  capable 
permanently  of  taking  the  place  of  the  normal  aerobic  pro- 
cess. Though  the  stoppage  of  oxygen  can  be  to  a  certain 
extent  compensated  for,  the  vital  mechanism  gradually 
becomes  exhausted,  and  life  ceases  if  the  cessation  of  the 
supply  is  prolonged.  In  the  higher  plants  anaerobic  is  at 
the  best  only  capable  of  supplementing  aerobic  respiration, 
and  that  for  but  a  limited  period.  The  commencing 
asphyxiation  serves  as  a  stimulus  to  the  protoplasm,  which 
responds  by  setting  up  the  anaerobic  changes,  but,  like  all 
stimulations,  the  ultimate  effect  is  exhaustion  and  a  failure 
to  continue  the  response. 

There  are  other  plants,  however,  which  do  not  require 
oxygen  for  their  vital  processes,  and  accordingly  do  not 
absorb  it ;  indeed  many  of  them  are  incapable  of  carrying 
on  their  life  in  the  presence  of  oxygen.  They  are  of  a  very 
humble  type,  and  occur  only  among  the  Bacteria  and 
Fungi.  An  instance  may  be  found  in  the  organisms  which 
induce  the  formation  of  butyric  acid  from  sugar  or  lactic 
acid.  If  a  few  of  these  are  sown  in  a  suitable  liquid,  and 
this  is  then  enclosed  in  a  hermetically  sealed  flask  from 
which  free  oxygen  has  been  removed,  they  multiply  with 
extreme  rapidity,  until  indeed  either  their  food  supply  is 
exhausted,  or  the  waste  products  of  their  metabolism 
accumulate  to  an  inhibitory  extent.  If  a  little  free  oxygen 
is  admitted  their  activity  ceases  and  death  ensues,  or  they 
pass  into  a  resting  condition,  which  lasts  as  long  as  oxygen 
is  present.  We  must  not,  however,  necessarily  conclude 
that  their  metabolism  is  of  a  totally  different  kind  from 
that  of  others,  but  rather  that  they  set  up  the  decomposi- 
tion and  reconstruction  of  their  protoplasm  in  a  different 
way  from  those  plants  which  need  a  supply  of  oxygen  to 
determine  them. 


310  VEGETABLE  PHYSIOLOGY 


CHAPTEK  XIX 

GROWTH 

In  studying  the  growth  of  plants  we  must  bear  in  mind 
the  relation  which  it  bears  to  the  processes  of  metabolism 
which  we  have  already  discussed.  We  have  seen  that  the 
constructive  processes,  partly  anabolic  and  partly  kata- 
bolic,  are  much  greater  than  those  which  lead  to  the  dis- 
appearance of  material  from  the  plant-body.  The  result 
of  this  is  that  there  is  a  conspicuous  increase  in  the  substance 
of  the  plant,  as  well  as  an  accumulation  of  potential  energy 
which  can  be  made  use  of  by  the  plant  through  various 
decompositions  which  its  protoplasm  can  set  up.  The 
great  permanent  accumulation  of  material  is  what  we 
associate  with  the  processes  of  growth.  Here,  however, 
we  must  distinguish  between  the  increase  of  the  living 
substance,  which  is  essentially  an  anabolic  process,  and 
that  of  the  manufacture  of  the  framework,  the  construction 
of  cellulose,  wood,  cork,  and  other  products,  which  is  the 
result  of  katabolism. 

The  growth  of  the  living  substance  is  always  the  result 
of  constructive  metabolism,  and  is  attended  by  an  increase 
of  bulk  and  weight.  The  growth  of  an  organ  sometimes 
appears  to  be  independent  of  such  increase  of  weight : 
indeed,  a  diminution  of  the  weight  of  the  whole  structure  is 
sometimes  noticeable.  For  instance,  in  the  case  of  a  potato 
tuber  allowed  to  germinate  under  such  conditions  as  prevent 
the  absorption  of  food  materials  from  without,  we  meet 
with  a  marked  change  of  form ;  but,  owing  to  the  loss  of 


GKOWTH 


311 


moisture  by  transpiration,  and  of  carbon  dioxide  as  a 
consequence  of  its  respiration  or  the  katabolic  processes 
going  on  in  its  tissues,  the  resulting  plant  weighs  much 
less  than  the  original  potato. 
This  difference  is  how- 
ever rather  apparent  than 
real.  We  shall  see  that 
the  actual  growth,  as  well 
as  the  manufacture  of 
new  cells,  is  confined  to 
certain  regions.  In  these 
regions  there  is  a  consider- 
able increase  in  bulk  and 
weight,  but  as  the  materials 
which  are  used  for  the  pur- 
poses of  this  local  growth 
are  derived  from  substances 
stored  up  in  the  body  of 
the  tuber,  the  latter,  the 
greater  part  of  which  is 
not  at  any  time  the  seat 
of  the  growth,  diminishes 
in  weight  and  size  to  such 
an  extent  as  more  than  to 
counterbalance  the  gain  in 
the  growing  regions.  Hence 
the  whole  plant  weighs  less 


than  the  tuber,  though  COn-      FIG.    133.  —  LONGITUDINAL    SECTION    OF 
.  ,        ,  ,  . -,  -,  YOUNG  ROOT,  SHOWING  STRUCTURE  OF 

siderable  growth  may  have       GROWING  POINT,    x  20. 


1,  zone  of  cell  division;  2,  region  of 
greatest  growth ;  3,  region  of  complete 
differentiation. 


taken  place. 

Mere  increase   of   weight 
in  an  organ   does  not,    on 

the  other  hand,  necessarily  imply  any  growth.  The  depo- 
sition of  reserve  materials  in  many  seeds  does  not  take  place 
to  any  great  extent  till  their  mature  dimensions  are  reached, 
and  growth  is  therefore  completed.  It  involves,  however, 
a  considerable  increase  of  weight. 


312 


VEGETABLE  PHYSIOLOGY 


Growth  is  in  the  strict  sense,  then,  always  associated 
with  the  formation  of  new  living  substance,  and  is  very 
generally  accompanied  or  immediately  followed  by  additions 
to  the  framework  of  the  growing  cells  or  organs.  It  is  in 
nearly  all  cases  attended  by  a  permanent  change  of  form. 
This  is  perhaps  not  so  evident  in  the  case  of  axial  organs 
as  it  is  in  that  of  leaves  and  their  modifications,  though 
even  in  them  it  can  be  detected  to  a  certain  extent.  It  is 
much  more  conspicuous  in  the  case  of  leaves,  for  the  latter, 
as  they  expand  from  the  bud,  have  usually  a  different 
shape  from  that  of  the  adult  ones,  and  the  assumption  of 


FIG.  134. — SECTION  OF  BLADE  OF  LEAF,  SHOWING  THE  IRREGULAR  CELLS  OF 
THE  SPONGY  MESOPHYLL  ABUTTING  ON  THE  LOWER  EPIDERMIS. 

the  mature  form  is  a  gradual  process,  taking  place  as  the 
age  of  the  leaf  increases. 

This  change  of  form  can  be  seen  not  only  in  the  case 
of  an  organ  such  as  a  leaf,  but  also  in  that  of  the  indi- 
vidual cells  of  which  a  plant  consists.  In  the  apical  meristem 
of  the  root  of  a  flowering  plant  the  cells  when  first  formed 
are  almost  cubical  (fig.  183) ;  after  a  little  while  we  find 
many  of  them  becoming  elongated,  and  ultimately  prosen- 
chymatous.  Many  other  cases  can  be  noted,  particularly  the 
irregularly  shaped  cells  of  the  spongy  parenchyma  of  leaves 
(fig.  134),  the  stellate  cells  of  the  pith  of  certain  rushes 
(fig.  135),  the  laticiferous  cells  of  the  Spurges,  &c. 

Growth  may,  in  the  light  of  the  considerations  just 
advanced,  be  defined  as  permanent  increase  of  bulk,  attended 


GEOWTH 


313 


by  permanent  change  of  form.  It  is  the  process  of  passing 
from  the  embryonic  to  the  adult  condition.  We  must  not 
assume  that  every  increase  of  bulk  is  necessarily  growth  ; 
for,  as  we  shall  see,  in  growing  cells  and  members  there  is 
a  constant  stretching  of  the  cell  or  tissue  by  hydrostatic 


FIG.  135. — PORTION  OF  SECTION  OF  STEM  OF  RUSH,  SHOWING  STELLATE 
TISSUE  OF  THE  PITH,  WITH  LARGE  INTERCELLULAR  SPACES. 


pressure  or  turgidity,  which  can  be  distinguished  from 
growth  by  the  fact  that  it  can  be  removed,  the  result  being 
a  certain  diminution  of  the  size  of  the  part  under  consideration. 
Growth  in  the  lowliest  plants  may  be  co-extensive  with 
the  plant-body.  In  all  plants  of  any  considerable  size, 
however,  it  is  localised  in  particular  regions,  and  in  them 


314  VEGETABLE  PHYSIOLOGY 

it  is  associated  with  the  formation  of  new  protoplasts.  We 
have  already  mentioned  that  in  the  sporophytes  of  all 
the  higher  plants  there  exist  certain  regions  in  which  the 
cells  are  merismatic — that  is,  which  have  the  power  of 
cell-multiplication  by  means  of  division.  In  such  regions, 
when  a  cell  has  reached  a  certain  size,  which  varies  with 
the  individual,  it  divides  into  two,  each  of  which  increases 
to  the  orginal  dimensions  and  then  divides  again.  These 
regions  have  heen  called  growing  points  (fig.  133)  ;  they 
may  be  apical  or  intercalary.  In  such  stems  and  roots  as 
grow  in  thickness  there  are  other  growing  regions,  which 
consist  of  cylindrical  sheaths  known  as  cambium  layers  or 
phellogens.  By  the  multiplication  of  the  protoplasts  in 
these  merismatic  areas  the  substance  of  the  plant  is  increased. 
In  other  words,  as  these  growing  regions  consist  of  cells, 
the  growth  of  the  entire  organ  or  plant  will  depend  on  the 
behaviour  of  the  cells  or  protoplasts  of  which  its  merismatic 
tissues  are  composed. 

The  growth  of  such  a  cell  will  be  found  to  depend  mainly 
upon  five  conditions  :  (1)  There  must  be  a  supply  of  nutritive 
or  plastic  materials,  at  the  expense  of  which  the  increase  of 
its  protoplasm  can  take  place,  and  which  supply  the  needed 
potential  energy.  (2)  There  must  be  a  supply  of  water  to 
such  an  extent  as  to  set  up  a  certain  hydrostatic  pressure  in 
the  cell.  This  condition  we  have  already  considered  in  an 
earlier  chapter,  in  which  we  discussed  the  relation  of  proto- 
plasm to  water.  (3)  The  supply  of  water  must  be  associated 
with  the  formation  of  osmotic  substances  in  the  cell,  or  it 
cannot  be  made  to  enter  it.  In  the  absence  of  the  turgescence, 
which  will  be  the  result  of  the  last  two  conditions,  no  growth 
is  possible  for  reasons  that  will  presently  appear.  (4)  The 
cell  must  have  a  certain  temperature,  for  the  activity  of  a 
protoplast  is  only  possible  within  particular  limits,  which 
differ  in  the  cases  of  different  plants.  (5)  There  must  be  a 
supply  of  oxygen  to  the  growing  cell,  for,  as  we  have  seen,  the 
protoplast  is  dependent  upon  this  gas  for  the  performance 
of  its  vital  functions,  and  particularly  for  the  liberation  of 


GKOWTH 


315 


the  energy  which  is  demanded  in  the  constructive  processes. 
This  is  evident  also  from  the  consideration  that  the  growth 
of  the  cells  is  attended  by  the  growth  in  surface  of  the  cell- 
wall,  and  as  the  latter  is  a  secretion  from  the  protoplasm — 
a  product,  that  is,  of  its  katabolic  activity — such  a  decom- 
position cannot  readily  take  place  unless  oxygen  is  admitted 
to  it. 

Growth,  so  far  as  it  implies  only  the  formation  of  living 
substance,  is  thus  a  constructive  process.  It  is,  however, 
intimately  associated  with  destructive  metabolism  or  kata- 
bolism,  the  latter  being  involved  in  the  construction  of 
the  increased  bulk  of  the  framework  of  the  cell  or  cells, 
and  being  essential  to  supply  the  energy  needed  for  the 
constructive  processes. 

When  the  conditions  mentioned  are  present,  the  course 
of  the  growth  of  a  cell  appears  to  be  the  following  :  the 
young  cell,  immediately  it  is  cut  off  from  its  fellows,  absorbs 
water  in  consequence  of  the  presence  in  it  of  osmotically 
active  substances.  With  the  water 
it  takes  in  the  various  nutritive 
substances  which  the  former  con- 
tains in  solution.  There  is  set  up  at 
once  a  certain  hydrostatic  pressure 
due  to  the  turgidity  which  ensues 
upon  such  absorption,  and  the  ex- 
tensible cell- wall  stretches,  at  first, 
in  all  directions.  The  growth  of 
the  protoplasm  at  the  expense  of 
the  nutritive  matter  for  a  time  keeps 
pace  with  the  increased  size  of  the 
cell,  but  by  and  by  it  becomes  vacuo- 
lated  as  more  and  more  water  is 
attracted  into  the  interior.  Even- 
tually the  protoplasm  usually  forms 
only  a  lining  layer  to  the  cell-wall,  and  a  large  vacuole 
filled  with  cell-sap  occupies  the  centre  (fig.  136).  The 
growth  of  the  protoplasm,  though  considerable,  is  therefore 


FIG.  136.— ADULT  VEGETABLE 

CELLS.       x    500.       (After 

Sachs.) 
h,  ci  11- wall ;     p,  protoplasm  ; 

k  k,  nucleus,  with  nucleoli ; 

s  s',  vacuoles. 


316  VEGETABLE  PHYSIOLOGY 

not  commensurate  with  the  increase  in  the  size  of  the  cell. 
The  stretching  of  the  cell- wall  by  the  hydrostatic  pressure  is 
fixed  by  a  secretion  of  new  particles  and  their  deposition 
upon  the  original  wall,  which  as  it  becomes  slightly  thicker 
is  capable  of  still  greater  extension,  much  in  the  same  way 
as  a  thick  band  of  india-rubber  is  capable  of  undergoing 
greater  stretching  than  a  thin  one.  The  increase  in  surface 
of  the  cell- wall  is  thus  due,  firstly,  to  the  stretching  caused 
by  turgidity,  and,  secondly,  to  the  formation  and  deposition 
of  new  substance  upon  the  old.  The  latter  only  is  permanent ; 
the  former  can  be  removed  by  irrigating  the  cell  with  a 
solution  of  a  substance,  such  as  common  salt,  which  will 
rob  it  of  the  water  it  contains.  The  constructive  changes 
leading  to  the  formation  of  new  protoplasm  are  attended  in 
this  process  by  the  katabolic  formation  of  cell- wall  and  other 
substances,  such  as  the  osmotic  bodies  which  are  necessary 
to  draw  the  water  into  the  cell.  The  supply  of  oxygen 
is  needed  to  allow  the  protoplasm  to  undergo  these  kata- 
bolic decompositions,  enabling  it  thus  to  prepare  the  several 
products  spoken  of,  and  to  gain  from  such  decompositions 
the  energy  which  must  be  expanded  upon  the  construction 
and  reconstruction  of  the  living  substance,  and  used  in 
the  secondary  chemical  changes  which  supervene. 

The  process  of  the  growth  of  a  cell  is  limited  in  its  extent, 
though  the  limits  vary  very  widely  in  different  cases.  In 
some,  cells  grow  only  to  a  few  times  their  original  dimensions  ; 
in  others,  they  may  attain  a  very  considerable  size.  In  any 
case,  however,  we  can  notice  that  the  rate  of  growth  takes 
a  certain  course  throughout  the  process  ;  it  begins  slowly, 
increases  to  a  maximum,  and  then  becomes  gradually 
slower  till  it  stops.  The  time  during  which  these  regular 
changes  in  the  rate  can  be  observed  is  generally  spoken 
of  as  the  grand  period  of  growth. 

Changes  in  the  shapes  of  cells  arising  during  growth 
depend  upon  two  factors.  The  capacity  of  the  cell  to  yield 
to  hydrostatic  pressure  may  be  affected  differently  in  different 
directions  by  the  conditions  of  the  cells  which  surround  it. 


GEOWTH  317 

In  the  merismatic  tissue  of  a  growing  point  there  is  generally 
least  resistance  on  the  side  of  the  free  apex  of  the  organ, 
and  hence  an  increased  protrusion  of  the  latter  results. 
Whatever  may  be  the  distribution  of  such  pressure  the 
growth  of  the  cell  will  be  greatest  in  the  line  of  least  resist- 
ance. If  any  internal  cause  should  give  rise  to  differences 
in  the  uniformity  of  hydrostatic  pressure  in  all  directions, 
the  growth  will  be  most  extensive  in  the  line  of  the  greatest. 
In  the  second  place  the  extensibility  of  the  cell- wall  may  be 
locally  modified  by  the  protoplasm,  so  that  a  uniform 
internal  hydrostatic  pressure  may  affect  one  part  more  than 
another,  and  the  growth  consequently  become  irregular, 
giving  rise  in  many  cases  to  cells  of  curious  form. 

If  we  consider  the  behaviour  of  a  growing  organ  in  the 
light  of  these  facts,  we  shall  see  that,  like  the  cell,  it  must 
show  a  grand  period  of  growth.  If  we  take  the  case  of  a 
root,  in  which  the  changes  can  be  traced  most  easily  on 
account  of  the  simplicity  of  its  structure,  we  find  that  just 
behind  the  apex  the  cells  are  all  in  active  division.  Growth 
is  small  and  consists  mainly  in  an  increase  of  the  quantity 
of  protoplasm,  for  the  cells  divide  again  as  soon  as  they 
have  reached  a  certain  size.  As  new  cells  are  continually 
formed  in  the  merismatic  mass,  those  which  are  farthest 
from  the  apex  gradually  cease  to  divide  and  a  different 
process  of  growth  takes  place  in  them,  which  is  associated 
more  particularly  with  the  formation  of  the  vacuoles  and 
consequently  with  the  establishment  of  considerable  hydro- 
static pressure,  thus  causing  the  bulk  of  the  cells  to  be  greatly 
enlarged,  as  we  have  described.  Hence  it  is  here  that  the 
actual  extension  in  length  of  the  root  goes  on,  and  the 
cells  reach  the  maximum  point  of  the  grand  period.  They 
then  gradually  lose  the  power  of  growth,  the  oldest  ones  or 
those  farthest  from  the  apex  parting  with  it  first,  and  they 
pass  slowly  over  into  the  condition  of  the  permanent  tissue 
(fig.  133).  In  this  way  each  zone  of  the  root  which  may  be 
distinguished  goes  through  a  grand  period  of  growth.  At 
first  when  the  cells  are  merismatic,  growth  is  at  a  minimum, 


318  VEGETABLE  PHYSIOLOGY 

it  gradually  becomes  accelerated,  reaches  a  maximum,  and 
slowly  ceases,  exactly  as  did  that  of  the  cell  which  we  first 
considered.  By  careful  examination  of  a  growing  root  it 
can  be  found  that  the  growth  is  greatest  just  behind  the 
merismatic  region.  If  a  young  root  be  taken  and  marked 
out  into  zones  by  a  series  of  short  lines  at  equal  distances 
apart  (fig.  137,  A),  and  then  allowed 
to  continue  its  growth,  it  will  be 
found  that  the  lines  remain  close 
together  at  the  apex  and  for  a  very 
short  distance  from  it.  Then  they 
become  separated  by  broader  spaces 
(fig.  137,  B).  Farther  back  still  the 
FIG.  is:.  — GERMINATING  original  intervals  between  the  lines 

BEAN,  SHOWING  GROWTH  .„  . 

OF  THE  RADICLE.  will   again   be   found   to   be   almost 

unaltered.  The  second  region  corre- 
sponds to  the  part  where  the  cells  are  undergoing  the  enlarge- 
ment described.  The  total  growth  of  the  root  is,  of  course, 
the  sum  of  the  increments  of  all  the  zones  so  marked  out. 

The  same  order  of  events  may  be  ascertained  to  take 
place  in  the  stem,  but  in  this  region  it  is  complicated  by 
the  occurrence  of  nodes  and  internodes.  Growth  in  length 
is  almost  confined  to  the  latter,  each  of  which  passes  through 
a  similar  grand  period.  The  growth  of  the  stem  is  the 
algebraical  sum  of  the  growth  of  the  internodes,  many  of 
which  may  be  growing  simultaneously  and  which  will  be  at 
any  particular  moment,  therefore,  at  different  parts  of  their 
grand  period.  The  region  of  growth  in  the  stem  is,  as  a 
rule,  much  longer  than  that  in  the  root. 

The  growth  of  the  leaf  shows  a  little  variation.  The 
apical  growth,  as  a  rule,  is  not  very  long  continued,  and 
the  subsequent  enlargement  of  the  leaf  is  due  to  an  inter- 
calary growing  region  near  the  base.  This  area  has  the 
merismatic  cells  at  about  its  centre,  and  regions  of  greatest 
growth  are  on  both  sides  of  it.  This  can  be  traced  more 
easily  in  the  elongated  leaves  of  Monocotyledons  than  in 
those  of  Dicotyledons. 


GEOWTH 


819 


The  grand  period  itself  is  not  quite  uniform,  as  the  rates 
of  growth  in  the  active  region  may  and  do  vary  with  changes 
in  external  conditions,  and  with  differences  in  activity  in 
the  protoplasm  from  time  to  time.  This  can  be  observed 
very  favourably  in  the  case  of  a  growing  stem,  which  shows 
considerable  differences  in  its  rate  of  growth  during  twenty- 
four  hours.  The  growth  is  greatest  during  the  night  and 
least  during  the  day,  and  the  variations  in  the  rate  are 
fairly  regular,  the  total  growth  during  successive  periods 
of  twenty-four  hours  being,  on  the  whole,  uniform.  This 
regular  variation  of  the  rate  constitutes  what  is  known 
as  the  daily  period  of  growth  in  length. 

An  instrument  by  which  the  progress  of  growth  of  such 
a  structure  as  a  stem  canjue 
ascertained  and  registered  is 
known  as  an  auxanometer. 
A  very  convenient  form, 
which  registers  the  gradual 
increase  in  length  automati- 
cally, has  been  constructed 
by  Pfeffer,  and  is  repre- 
sented in  fig.  138.  A  thread 
attached  to  the  plant  passes 
over  the  small  wheel  x, 
which  is  cemented  on  the 
large  wheel  r,  and  accurately 
centred  about  the  same  axis. 
A  thin  lever  z  is  attached 
to  another  thread  which  is 
passed  over  the  large  wheel, 
and  is  made  to  write  upon 
the  smoked  surface  of  a 
paper  fastened  round  the 
cylindrical  drum  t.  The 
string  is  kept  tight  by  the  counterbalancing  weight  g.  The 
drum  is  caused  to  rotate  slowly  upon  its  axis  by  clock- 
work, so  that  the  indicator  traces  a  line  along  its  surface. 


FIG.    138. — PFEFFER'S    AUTOMATICALLY 

REGISTERING     AUXANOMETER.        (After 

Detmer.) 


320  VEGETABLE  PHYSIOLOGY 

So  long  as  no  growth  takes  place  this  line  is  horizontal, 
but  as  the  indicator  is  displaced  downwards  by  the  descent 
of  the  small  weight  attached  to  the  first  cord,  which  is 
attendant  upon  any  elongation  of  the  axis  of  the  plant,  the 
line  actually  traced  during  growth  is  a  spiral.  The  rate 
of  the  drum's  revolution  being  known,  the  amount  of  the 
elongation  of  the  axis  per  hour  can  easily  be  calculated. 
The  actual  augmentation  of  the  plant's  axis  is  magnified 
in  the  record,  in  a  ratio  dependent  upon  the  ratio  between 
the  radii  of  the  large  and  small  wheels  r  and  x. 

For  the  sake  of  simplicity  of  description  it  has  been 
assumed,  in  what  has  already  been  said,  that  the  turgidity  of 
the  cells  in  the  growing  member  is  uniform.  This,  however, 
is  far  from  being  the  case.  There  is  generally  a  certain 
variation  in  this  turgidity  in  the  different  parts  of  the 
elongating  member.  The  simplest  case  which  we  may 
consider  is  one  which  shows  a  difference  in  structure  on  two 
sides  ;  such  a  member  is  described  as  dorsiventral.  The 
two  sides  will  often  show  a  difference  of  degree  of  tur- 
gidity and  consequently  of  rate  of  growth.  If  we  consider 
a  leaf  of  the  common  Fern,  we  find  that  in  its  young  con- 
dition it  is  closely  rolled  up,  the  upper  or  ventral  surface 
being  quite  concealed.  As  it  gets  older  it  gradually  unfolds 
and  expands  into  the  adult  form.  This  is  due  to  the  fact 
that  in  the  young  condition  the  turgidity  and  consequent 
growth  are  greater  on  the  dorsal  side  of  the  leaf,  so  that  it 
becomes  rolled  up  as  described.  As  it  gets  older  the  maxi- 
mum turgidity  and  growth  change  to  the  upper  side  and  so 
it  becomes  unfolded  or  expanded.  These  two  conditions 
are  generally  described  under  the  names  of  hyponasty  and 
epinasty  respectively. 

These  conditions  are  not  confined  to  the  leaves  of  ferns, 
but  may  be  detected  in  those  of  other  plants,  though  to  a 
less  conspicuous  degree.  It  is  in  consequence  of  them  that 
the  leaves  of  the  bud  always  fold  over  the  apex  of  the  stem 
from  which  they  spring.  The  opening  and  closing  of  certain 
flowers,  such  as  the  Crocus,  depend  upon  similar  variations. 


GROWTH  321 

Cylindrical  organs  may  exhibit  similar  phenomena. 
One  side  of  the  cortex  of  a  stem  may  be  more  turgid  than 
the  rest,  and  the  maximum  turgidity  with  its  consequent 
growth  may  pass  a  little  later  to  the  opposite  side  and 
subsequently  alternate  between  them.  The  greater  turgidity 
of  the  cells  is  often  accompanied  by  an  increased  extensi- 
bility of  the  cell-walls  of  the  temporarily  more  turgid  region. 
The  growing  apex  will  thus  alternately  incline  first  to 
one  side  and  then  to  the  other,  exhibiting  a  kind  of  nodding 
movement  in  the  two  directions.  This,  known  as  nutation, 
is  of  frequent  occurrence,  particularly  in  such  stems  as  are 
slightly  flattened  instead  of  being  truly  cylindrical. 

The  region  of  maximum  turgidity  in  the  cortex  instead 
of  occurring  alternately  on  two  opposite  sides  may  pass 
gradually  and  regularly  round  the  growing  zone.  The 
apex  of  a  truly  cylindrical  stem  in  this  case  will  describe 
a  circle,  or  rather  a  spiral,  as  it  is  elongating  all  the  time, 
pointing  to  all  points  of  the  compass  in  succession.  This 
continuous  change  of  position  has  been  described  by  Darwin 
as  circumnutation,  and  has  been  said  by  him  to  be  universal 
in  all  cylindrical  growing  organs.  The  passage  of  the 
maximum  turgidity  round  the  stem  may  vary  in  rapidity 
at  different  places,  causing  the  circle  to  be  replaced  by  an 
ellipse.  Indeed  the  simplest  nutation  spoken  of  above  may 
be  regarded  as  only  an  extreme  instance  of  the  latter. 

The  variations  of  turgidity  which  cause  circumnutation 
only  affect  the  zone  of  active  growth.  They  are  not  observ- 
able towards  the  base  of  this,  so  that  the  adult  part  becomes 
straight,  and  growth  is  ultimately  in  a  straight  line. 

Circumnutation  is  exhibited  during  growth  also  by  the 
hyphse  of  many  fungi,  some  of  which  have  a  coenocytic 
structure.  In  these  cases  the  movement  appears  to  be  due 
to  a  rhythmic  variation  in  the  extensibility  of  the  membrane, 
induced  probably  by  the  protoplasm.  It  cannot  be  caused 
by  differences  of  turgidity  on  the  two  sides  of  the  hypha  as 
this  contains  only  one  cavity. 

By  these  movements  of  the  growing  apices — movements 


322  VEGETABLE   PHYSIOLOGY 

incident  to  growth,  and  proceeding  primarily  from  internal 
causes — many  advantages  are  secured  by  the  plant.  In 
the  case  of  a  climbing  stem,  the  circumnutation  enables  it 
to  reach  a  support,  round  which  it  twines,  so  that  with 
but  little  expenditure  of  substance  it  can  secure  access  to 
more  light  and  air  than  it  could  obtain  in  its  absence. 
Boots  by  the  same  method  are  enabled  more  easily  to  make 
their  way  through  the  crevices  of  the  soil.  The  embryo 
shows  in  one  or  other  of  its  parts  strong  hyponastic 
curvature,  forming  an  arch  which  enables  it  to  leave  the 
seedcoats  and  make  its  way  through  the  soil  without  damage 
to  the  young  delicate  plumule,  its  progress  being  helped 
by  simultaneous  circumnutation.  On  reaching  the  surface, 
epinastic  growth  causes  it  to  assume  an  erect  position,  the 
arch  opening  out  till  the  direction  of  growth  is  vertical. 
Coincidently  with  this  change,  circumnutation  of  the  apical 
region  replaces  that  of  the  portion  which  was  at  first  arched. 

During  the  period  of  growth  the  young  organ  is  extremely 
sensitive  to  changes  in  its  environment,  responding  to  such 
stimulating  influences  by  further  modifications  of  its  be- 
haviour. These  will  be  considered  in  detail  in  a  subsequent 
chapter. 

Besides  the  hydrostatic  tension  set  up  in  the  cells  of 
the  growing  regions,  the  processes  of  growth  are  accom- 
panied by  the  development  of  other  tensions  in  the  interior 
of  the  growing  member.  These  appear  to  depend  upon 
differences  between  the  turgidities  of  their  several  tissue 
systems  as  these  develop,  and  upon  different  rates  of  growth 
of  different  internal  parts.  If  a  petiole  of  Ehubarb  is 
taken,  and  a  thin  strip  is  peeled  from  one  side,  it  will 
immediately  curl  outwards.  If  it  is  then  placed  in  apposi- 
tion with  the  part  from  which  it  was  cut,  it  will  be  found 
to  be  appreciably  shorter  than  the  rest  of  the  petiole.  If 
the  petiole  is  carefully  measured,  and  then  deprived  of  its 
cortical  covering  by  the  separation  of  successive  strips, 
the  central  part  will  be  found  to  be  slightly  longer  than 
the  original  petiole.  In  such  a  petiole  the  central  part  is 


GEOWTH  823 

clearly  compressed  by  the  external  portions,  and  when 
these  are  removed  it  undergoes  an  extension  which  is  the 
expression  of  the  amount  of  such  compression.  Similarly 
the  external  parts  are  stretched  longitudinally  by  the 
central  region,  and  when  they  are  freed  from  it,  the  recoil 
is  accompanied  by  a  diminution  of  their  length.  There  is 
thus  a  longitudinal  tension  in  the  petiole,  due  to  the  greater 
turgescence  of  the  central  part,  which  stretches  the  outer 
portions,  and  is  itself  compressed  by  their  greater  rigidity 
resisting  the  hydrostatic  extension.  This  tension  is  not 
due  to  greater  growth,  but  to  more  pronounced  turgidity, 
for  if  such  a  petiole  is  soaked  for  a  time  in  salt  solution 
till  the  water  is  in  great  part  removed  from  its  interior,  and 
it  has  become  flaccid,  removal  of  the  cortex  is  not  accom- 
panied by  the  same  changes  of  dimension.  A  similar  ex- 
periment may  be  performed  on  the  hollow  flower-stalk  of 
a  Dandelion.  If  it  is  slit  into  two  halves  by  a  vertical 
cut,  the  two  parts  curl  outwards  from  each  other,  showing 
a  similar  tension  in  the  internal  regions. 

Transverse  tensions  in  young  growing  axes  can  also  be 
demonstrated.  The  cortex  is  found  to  be  strained  outwards 
by  the  central  tissues,  so  that  if  a  ring  of  it  is  cut  out  of 
such  an  axis  and  split  longitudinally,  it  shortens.  If  the 
split  ring  is  again  put  back  in  its  original  position,  it  will 
not  completely  surround  the  stem.  The  central  tissues 
are  in  a  state  of  compression,  and  the  cortex  is  one  of 
extension,  laterally  as  well  as  longitudinally,  as  in  the 
other  case  already  quoted. 

Transverse  tensions  of  a  similar  kind  are  set  up  in  the 
course  of  the  thickening  of  stems  and  roots  by  the  activity 
of  the  cambium  layer,  by  the  division  of  whose  cells  new 
bast  is  formed  behind,  and  new  wood  in  front  of  it.  The 
bast  and  cortex  are  thus  compressed  outwards,  and  the 
wood  and  pith  inwards,  on  account  of  the  formation  of  the 
new  material.  The  phellogens,  which  form  rings  of  cork  at 
various  depths  in  the  cortex,  give  rise  to  similar  strains. 
Sheaths  of  new  cells  are  intercalated  in  the  substance  of 

21* 


324  VEGETABLE   PHYSIOLOGY 

the  delicate  tissue,  which  thus  becomes  greatly  thickened. 
These  tensions  are  due  to  growth,  and  not,  like  the  others, 
to  turgidity  of  the  tissues.  They  cannot  consequently  be 
removed  by  treatment  with  salt  solutions. 

These  tensions  are  capable  of  demonstration  all  through 
the  life  of  such  stems  and  roots  as  increase  in  thickness. 
They  give  us  a  partial  explanation  of  the  structure  of  the 
annual  rings  of  wood  which  are  exhibited  by  such  stems  and 
roots,  and  of  the  ruptures  that  are  generally  noticeable  in 
the  exterior  of  such  parts. 

In  the  absence  of  various  external  stimulating  influences, 
which  will  be  discussed  later,  young  growing  members  show 
a  tendency  to  elongate  uniformly,  so  that  the  direction 
of  their  growth  is  a  straight  line.  Though  the  apex  of  any 
of  them  may  continually  show  the  movement  of  circum- 
nutation,  the  mature  part  generally  takes  up  a  fixed  position, 
growing  vertically  or  horizontally  as  the  case  may  be. 
This  position  is,  however,  usually  due  to  the  combined 
action  of  a  number  of  external  forces  acting  upon  the 
growing  member.  The  inherent  tendency  just  spoken  of 
can  be  satisfactorily  seen  when,  by  artificially  eliminating 
the  action  of  such  forces,  the  plant  is  not  exposed  to  their 
stimulating  influences.  Such  a  tendency  has  been  called 
Rectipetality.  It  becomes  apparent  also  in  the  case  of  a 
member  which  has  become  curved,  owing  to  the  action  of 
one  or  other  of  the  stimulating  influences  referred  to.  If 
it  is  removed  from  the  influence  of  the  stimulus,  it  becomes 
straight  again. 


325 


CHAPTEK  XX 

TEMPERATURE    AND   ITS    CONDITIONS 

The  various  processes  which  are  characteristic  of  vegetable 
life  only  take  place  so  long  as  the  plant  is  exposed  to  a 
particular  range  of  temperature,  which  lies  between  the 
freezing  point  of  water  and  about  50°  C.,  a  few  exceptions 
on  both  sides  of  that  range,  however,  being  met  with.  It 
is  consequently  essential  to  the  well-being  of  the  organism 
that  its  temperature  shall  be  maintained  within  those 
limits.  While  life  is  possible  within  this  range  it  is  not 
equally  well  manifested  at  all  the  points  which  lie  between 
the  limits  ;  each  vital  function  indeed  shows  considerable 
variation  in  this  respect.  There  is  a  certain  point,  lying 
generally  near  the  freezing  point,  below  which  it  cannot  be 
observed.  There  is  another  point  near  the  upper  limit, 
beyond  which  it  is  not  carried  out,  and  somewhere  between 
them  there  is  a  point  at  which  it  is  manifested  most  advan- 
tageously. These  three  points  are  known  respectively  as 
the  minimum,  maximum,  and  optimum  temperatures  for 
that  function.  These  temperatures  vary  for  each  function 
which  accompanies  the  life  of  any  particular  plant.  They 
are  not,  moreover,  in  the  case  of  a  particular  function, 
necessarily  identical  in  different  plants. 

The  process  of  photosynthesis,  for  instance,  commences 
in  the  grasses  at  about  2°  C.,  while  in  the  Potamogetons  it 
cannot  be  detected  below  10°  C.  The  absorption  of  water 
by  the  roots  of  the  Turnip  and  other  cruciferous  plants  may 


326  VEGETABLE  PHYSIOLOGY 

begin  when  the  soil  has  a  temperature  but  slightly  above 
the  freezing  point  of  water  ;  in  the  case  of  the  Tobacco- 
plant  it  must  be  at  12°  C.  at  least.  The  lowest  temperature 
for  the  germination  of  the  seed  varies  between  5°  C.  for  the 
Wheat  and  13°  C.  for  the  Vegetable  Marrow.  The  upper 
limit  for  this  function  in  the  cases  of  these  two  plants  has 
been  ascertained  to  be  37°  C.  for  the  former  and  42°  C. 
for  the  latter.  The  optimum  point  for  the  growth  of  the 
roots  of  a  seedling  of  Maize  is  27°  C.,  while  the  correspond- 
ing temperature  for  that  of  the  Barley  and  Wheat  is  about 
23°  C.  Kespiration  seems  to  show  similar  limits,  but  very 
few  observations  have  been  made  upon  it  from  this  point 
of  view.  The  optimum  appears  to  be  a  little  over  30°  C., 
and  the  maximum  25  degrees  higher. 

The  temperature  of  a  terrestrial  plant  is  subject  to 
great  and  frequent  fluctuations,  and  there  is  considerable 
difficulty  in  securing  for  it  for  any  length  of  time  the 
optimum  temperature  for  any  of  its  vital  functions,  and 
indeed  sometimes  of  maintaining  it  within  the  limits 
which  are  essential.  As  a  rule  such  a  plant  only  secures  a 
general  approximation  to  the  optimum  point.  The  difficulty 
is  due  to  the  fact  that  there  is  a  continual  and  yet  vari- 
able interchange  of  heat  between  itself  and  its  environment. 
During  the  daytime  it  is  constantly  receiving  supplies  of 
radiant  energy  from  the  sun,  and  as  the  air  surrounding  it 
becomes  warmer,  a  certain  though  much  smaller  amount 
is  absorbed  by  conduction.  It  is  further  continually  ex- 
pending heat  on  the  maintenance  of  transpiration,  losing 
it  also  from  time  to  time  by  radiation  and  conduction. 
In  its  own  metabolic  processes  it  is  sometimes  rendering 
heat  latent,  and  always  liberating  it  by  the  processes  of 
respiration,  fermentation,  &c.  Naturally,  its  temperature 
relationships  are  continually  varying.  On  the  whole,  such 
a  plant  tends  to  approximate  its  temperature  to  that  of 
its  environment,  but  an  equalisation  is  seldom  reached, 
as  both  are  varying  simultaneously,  and  owing  to  the 
slowness  of  the  conduction  of  heat  along  vegetable  tissues, 


TEMPERATUKE  AND  ITS  CONDITIONS        327 

the  processes  of  adjustment  only  take  place  with  difficulty. 
The  trunk  of  n  tree  is  during  the  day  often  cooler  than 
the  air  and  warmer  than  the  latter  during  the  evening 
and  night.  The  mean  annual  temperature  of  such  a  tree 
trunk  is,  however,  about  equal  to  that  of  the  air.  Less 
bulky  parts  than  the  trunk,  the  leaves  for  instance,  are 
very  often  much  cooler  than  the  air.  This  is  made  evident 
by  the  frequency  with  which  dew  or  even  hoar-frost  may 
be  detected  on  their  surfaces.  A  thermometer  placed  upon 
grass  often  gives  a  much  lower  reading  than  one  suspended 
in  the  air  a  little  above  the  ground.  This  is,  no  doubt,  due 
to  the  loss  of  heat  by  radiation  from  the  leaves.  Boots 
are  often  cooler  than  the  air,  losing  heat  by  conduction  to 
the  soil,  and  by  the  evaporation  which  takes  place  into 
their  intercellular  spaces. 

Aquatic  plants  are  less  subject  to  these  disturbances 
than  terrestrial  ones.  The  range  of  temperature  of  the 
water  surrounding  them  is  smaller,  and  as  they  are  practic- 
ally in  contact  with  water  within  and  without,  the  internal 
changes  of  temperature  incident  to  their  metabolism  are 
much  more  readily  equalised. 

In  discussing  the  changes  of  temperature  in  the  body 
of  a  plant  we  have  to  deal  at  the  outset  with  the  supplies 
of  heat  which  it  receives.  We  have  already  examined 
them  from  the  point  of  view  of  the  absorption  of  energy 
from  without,  but  we  may  pursue  still  further  here  the 
question  of  the  warming  or  cooling  of  the  plant  itself  during 
such  absorption. 

The  chief  source  from  which  heat  is  derived  is  the 
radiant  energy  of  the  sun.  When  bright  sunshine  falls 
upon  a  leaf  about  a  quarter  of  its  radiant  energy  is  absorbed. 
A  much  larger  relative  amount  is  taken  up  when  the 
light  is  less  bright ;  in  a  strong  diffuse  light,  such  as  that 
from  a  clear  northern  sky,  the  absorption  amounts  to  about 
96  per  cent,  of  the  incident  energy.  We  cannot  at  present 
discriminate  with  any  accuracy  between  the  influence  of 
the  heat  rays  and  that  of  those  of  the  other  parts  of  the 


328  VEGETABLE  PHYSIOLOGY 

spectrum.  No  doubt  the  relative  proportions  vary  con- 
siderably during  the  year. 

This  radiant  energy  falling  upon  the  leaf  comes  into 
relationship  with  it  quite  independently  of  the  temperature 
of  the  air  through  which  the  rays  pass,  though  the  latter 
gradually  rises  also,  particularly  during  bright  sunshine. 
As  we  have  seen  already,  three-quarters  of  the  radiant 
energy  during  such  conditions  is  not  absorbed  by  the 
plant.  The  temperature  of  the  air  round  the  leaves  under 
a  diffuse  illumination  rises  but  slightly,  as  only  about 
four  per  cent,  of  the  radiant  energy  falling  upon  them 
remains  unabsorbed  by  them 

In  certain  cases,  particularly  where  the  temperature  of 
the  air  remains  low  for  considerable  periods,  as  in  high 
latitudes  and  on  mountains,  many  of  the  light  rays  appear 
to  be  transformed  into  heat.  These  are  the  rays  which 
are  most  vigorously  absorbed  by  chlorophyll  and  by 
anthocyan,  and  which  cause  the  fluorescence  of  those 
pigments.  The  importance  of  anthocyan  in  this  respect 
especially  may  be  noticed.  It  is  of  very  frequent  occur- 
rence among  plants  which  grow  in  deep  shade,  and  which 
receive  accordingly  but  little  radiant  energy.  It  is  usually 
found  on  the  under  side  only  of  foliage  leaves,  and  in  special 
leaves  produced  in  summer  upon  the  stems  of  deciduous 
shrubs  which  occur  upon  the  outskirts  of  forests,  or  in 
shady  spots  in  their  interior.  It  is  found  also  at  certain 
times  on  the  upper  sides  of  foliage  leaves,  particularly  when 
the  ordinary  sources  of  heat  are  deficient,  as  in  the  cold 
weather  of  early  spring.  Many  grasses  which  when  in  the 
lowlands  are  only  green  in  colour  develop  a  great  amount 
of  the  red  or  purple  anthocyan  when  they  grow  near  the 
snow-line. 

In  both  shady  and  alpine  habitats  the  function  of  the 
anthocyan  appears  to  be  the  same,  to  secure  to  the  plant 
a  certain  amount  of  heat  by  the  transformation  of  the 
light  rays. 

The  absorption  of  heat  from  the  environment  by  the 


TEMPERATURE  AND  ITS  CONDITIONS       829 

processes  of  conduction  is  particularly  noticeable  in  the 
case  of  aquatic  plants.  Indeed,  any  alteration  of  the  tem- 
perature of  either  the  plant  or  the  surrounding  water  is 
readily  transmitted  from  the  one  to  the  other.  Similar 
transmission  of  heat  from  the  soil  to  the  roots  can  take 
place,  and  no  doubt  has  a  considerable  effect  in  promoting 
the  well-being  of  the  latter,  which,  as  we  have  seen,  con- 
tinually lose  heat  by  the  evaporation  of  water  from  their 
cells  into  their  intercellular  spaces.  Here,  however,  as  in 
the  last  case,  the  conduction  of  heat  varies  in  direction 
according  to  the  relative  temperatures  of  soil  and  root. 

The  absorption  of  heat  from  the  air  in  contact  with  the 
general  surface  must  play  a  part  very  frequently  in  the  heat 
interchanges.  As  in  the  other  cases  mentioned,  however, 
the  direction  of  this  interchange  is  not  constant. 

While  we  can  thus  recognise  these  ultimate  sources  of 
heat  supply,  we  find,  no  less  evident,  certain  ways  in 
which  heat  is  given  off  by  the  plant  in  greater  or  less 
amount. 

Of  these  losses  the  first  and  most  important  is  the  expen- 
diture which  is  necessary  in  order  to  evaporate  the  water 
of  transpiration.  There  can  be  no  doubt  that  the  amount 
of  transpiration  is  very  largely  determined  by  the  amount 
of  the  sun's  rays  which  the  plant  receives.  Not  only 
are  its  stomata  open  widely  in  bright  light,  so  that  the 
vapour  can  readily  diffuse  into  the  air,  but  the  actual  eva- 
poration from  the  cells  into  the  intercellular  passages  is 
enormously  accelerated  during  the  absorption  of  the  radiant 
energy.  The  amount  of  the  latter  which  is  taken  up  by  a 
leaf  has  been  computed  to  be  nearly  fifty  times  the  amount 
which  can  be  utilised  in  the  process  of  photosynthesis  ;  if 
the  heat  were  allowed  to  accumulate  in  the  leaf  unchecked, 
it  has  been  calculated  that  its  temperature  would  rise  during 
bright  sunshine  at  the  rate  of  more  than  12°  C.  per  minute, 
with  of  course  very  rapidly  fatal  results.  What  is  not  used 
for  photosynthesis  is  employed  in  the  evaporation  of  the 
water  of  transpiration,  the  leaf  being  thus  kept  cool.  It 


330  VEGETABLE  PHYSIOLOGY 

is  noteworthy  that  whether  the  leaf  is  brightly  or  only 
moderately  illuminated  the  same  relative  proportions  of  the 
total  energy  absorbed  are  devoted  to  the  purposes  of  trans- 
piration and  photosynthesis. 

When  we  review  the  phenomena  of  transpiration  we  hnd 
two  very  important  considerations  presenting  themselves 
to  us.  On  the  one  hand,  the  suggestion  comes  that  the 
enormous  stream  of  water  passing  through  a  terrestrial 
plant  is  necessary  in  order  that  a  sufficient  amount  of  in- 
organic salts  may  be  supplied  to  the  leaves,  and  that  the 
process  of  transpiration  is  maintained  so  that  such  a  supply 
may  be  at  the  disposal  of  the  protoplasts.  The  dilute  solu- 
tions which  are  absorbed  naturally  involve  the  transport 
of  a  large  amount  of  water  with  the  salts.  Transpiration 
seems  thus  to  be  subordinate  to  food  supply. 

On  the  other  hand,  the  temperature  relations  which  we 
have  just  examined  appear  to  place  transpiration  upon 
quite  a  different  plane.  Instead  of  being  a  subordinate 
process,  it  appears  to  be  imperative  in  order  to  prevent  a 
fatal  rise  of  temperature  in  the  metabolic  protoplasts  ;  to 
be  concerned  primarily,  that  is,  in  the  regulation  of  the 
conditions  necessary  for  the  maintenance  of  metabolism 
and  life,  rather  than  in  the  supply  of  material  for  metabolic 
purposes. 

Which  of  these  is  the  chief  function  of  transpiration  will 
probably  depend  upon  circumstances.  The  process  serves 
the  two  purposes,  sometimes  one,  sometimes  the  other, 
being  the  more  prominent. 

Another  cause  of  loss  of  heat  is  found  in  radiation,  which 
takes  place  to  a  very  important  extent  from  the  surfaces 
of  flattened  organs  such  as  leaves.  This  radiation  is  to  a 
certain  extent  independent  of  the  temperature  of  the  sur- 
rounding air,  and  leads  in  some  cases  to  a  leaf  being  several 
degrees  cooler  than  the  latter.  A  thermometer  placed  on 
the  grass  will  frequently  show  a  temperature  some  nine  or 
ten  degrees  lower  than  another  one  suspended  a  few  inches 
above  the  surface  of  the  ground.  Evidence  of  the  activity 


TEMPERATURE  AND  ITS  CONDITIONS      381 

of  radiation  at  night  is  afforded  by  the  frequent  appearance 
of  dew  or  hoar-frost  on  the  leaves. 

The  effects  of  excessive  radiation  often  threaten  to  be 
disastrous,  and  have  led  to  the  development  of  many  pro- 
tective adaptations  by  various  plants.  The  masses  of 
woolly  hairs  which  are  often  found  upon  leaves,  forming, 
indeed,  in  some  cases  a  thick  mantle,  must  generally  be 
looked  upon  as  such  a  defensive  mechanism.  The  delicate 
leaves  of  buds  are  often  protected  by  thick  scale  leaves, 
which  in  some  cases  are  hairy,  in  others  furnished  with 
resinous  excretions,  to  serve  the  same  purpose.  No  doubt 
the  thick  cuticle  of  many  leaves  and  twigs  discharges  a 
similar  function. 

Some  plants  secure  a  protection  from  excessive  radiation 
from  the  upper  surfaces  of  the  leaves  during  the  night,  by 
folding  them  in  various  ways,  so  as  always  to  expose  as 
little  surface  as  possible,  and  that  surface  the  one  which  is 
least  susceptible  of  injury  by  cooling.  This  so-called  sleep 
or  nyctitropic  movement  plays  a  most  important  part  in 
the  retention  of  heat,  leaves  that  are  prevented  from  carry- 
ing it  out  perishing  very  rapidly.  The  features  of  this 
behaviour  will  be  examined  more  freely  in  a  subsequent 
chapter. 

Conduction  of  heat  from  the  plant  to  its  environment  is 
of  constant  occurrence,  but  it  is  exhibited  most  clearly 
by  plants  that  have  an  aquatic  habit.  The  general  inter- 
changes that  take  place  between  a  plant  and  the  water  in 
which  it  lives  range  usually  through  only  a  few  degrees  of 
temperature,  and  are  so  constantly  going  on  that  the 
temperature  of  both  tends  to  become  readjusted  after  every 
slight  disturbance.  In  some  cases,  however,  a  very  large 
amount  of  heat  is  dissipated  by  these  means,  as  we  may 
see  in  the  fermentation  of  a  saccharine  solution  by  yeast. 
The  metabolic  processes  of  the  latter,  incident  upon  its 
nutrition  and  respiration,  are  so  vigorous  that  a  very  large 
amount  of  energy  is  liberated  by  and  during  the  decom- 
position of  the  sugar,  and  this  takes  very  prominently  the 


332  VEGETABLE  PHYSIOLOGY 

form  of  heat  and  passes  from  the  plant  to  the  sugary  liquid 
in  which  it  lives. 

Terrestrial  plants  show  less  direct  evidence  of  the  loss 
of  heat  by  conduction.  Their  roots,  however,  no  doubt 
give  up  a  certain  amount  to  the  soil  at  different  times,  just 
as  at  others  they  absorb  heat  from  the  latter. 

When  we  compare  approximately  the  amount  of  heat 
absorbed  by  a  green  plant  with  that  which  is  given  off  by 
it,  we  find  that  in  all  cases  there  is  a  certain  excess  of  the 
former.  Most  plants  thus  show  a  certain  gain  of  heat 
from  their  environment.  This  does  not,  however,  usually 
manifest  itself  by  a  rise  of  temperature  in  the  tissues. 
There  is  no  uniformity  in  the  absorption  either.  At  times 
when  there  is  the  greatest  balance  in  favour  of  absorption 
throughout  the  whole  plant,  parts  of  it  may  be  giving  off 
considerable  quantities  and  may  be  cooler  than  the  average 
temperature  of  the  whole  plant. 

The  gain  of  heat  which  is  secured  in  this  way  is  to  be 
largely  regarded,  as  we  have  already  seen,  as  supplying 
energy  to  the  plant.  This  is  devoted  at  first  to  constructive 
processes,  and  thus  much  of  it  is  rendered  potential,  being 
afterwards  reconverted  into  the  kinetic  form  and  made  to 
reappear,  when  it  once  more  largely  takes  the  shape  of  heat, 
and  is  subsequently  devoted  to  purposes  of  growth,  meta- 
bolism, repair  of  cell-substance,  &c.,  as  we  have  already 
seen.  But  we  may  now  lay  a  certain  stress  on  the  fact  that, 
at  any  rate,  a  part  of  this  liberated  heat  is  devoted  to  a 
raising  of  the  temperature  of  the  cells  which  are  the  seat 
of  its  liberation. 

We  have  thus  an  elementary  though  very  incomplete 
mechanism  for  the  regulation  of  the  temperature  of  the 
plant.  An  excess  of  heat  is  absorbed  :  part  is  at  once 
applied  to  purposes  of  growth,  metabolism,  &c.  ;  part  is 
retained,  and  the  store  is,  as  it  were,  economised,  being 
liberated  later  with  some  reference  to  the  temperature  of 
the  parts  concerned  in  the  vital  processes. 

This  regulation  of  heat,  however,  is  very  rudimentary 


TEMPEKATUEE  AND  ITS  CONDITIONS       333 

and  imperfect.  We  do  not  find  that  an  increased  loss  of 
heat  stimulates  metabolism  in  such  a  way  as  to  set  up 
destructive  processes,  which  should  liberate  heat  to  com- 
pensate for  the  loss.  On  the  contrary,  such  increased  de- 
compositions are  promoted  by  a  rise  instead  of  a  fall  of 
temperature.  On  the  other  hand,  again,  the  processes  of 
growth,  repair,  and  constructive  metabolism  are  also  in- 
creased as  the  plant  becomes  warmer.  Conversely,  the 
setting  up  of  metabolic  activity  raises  temperature.  A  rise 
which  can  be  measured  by  a  delicate  thermopyle  follows 
the  cutting  or  wounding  of  a  potato,  or  the  bulb  of  an 
onion.  The  metabolism  set  up  is  chiefly  respiratory,  for  it 
is  accompanied  by  an  increased  output  of  carbon  dioxide. 

We  can  thus  speak  of  what  takes  place  as  a  tendency  to 
economise  and  distribute  heat,  rather  than  as  a  process  of 
regulation.  Even  the  distribution  of  heat,  whether  on  its 
first  absorption  or  after  subsequent  fixation  and  liberation, 
is  so  unequal  that  different  parts  of  a  plant  may  differ 
considerably  as  to  their  temperature. 

As  we  have  seen,  life  is  possible  within  certain  limits  of 
temperature  only.  The  maintenance  of  a  healthy  life 
depends  upon  the  adequate  discharge  of  various  functions, 
each  of  which  needs  again  a  certain  range.  The  limits 
within  which  life  is  possible  do  not  necessarily  coincide 
with  those  which  are  appropriate  to  every  function.  Out- 
side the  latter,  however,  a  plant  becomes  unhealthy  and 
eventually  perishes,  falling  a  victim  to  the  attacks  of  internal 
or  external  adverse  influences. 

We  do  not  find  that  all  plants,  or  indeed  all  parts  of 
plants,  show  the  same  amount  of  resistance  to  the  extremes 
of  heat  and  cold.  The  injury  which  any  part  of  a  plant 
experiences  under  such  conditions  depends  very  much 
upon  the  amount  of  water  which  it  contains.  If  more  than 
a  trace  of  the  latter  is  present,  the  formation  of  ice  which 
takes  place  below  0°  C.  may  lead  to  rupture  of  the  cells,  the 
ice  being  usually  deposited  outside  them.  A  considerable 
disturbance  of  the  osmotic  equilibrium  of  the  sap  may  occur, 


334  VEGETABLE  PHYSIOLOGY 

setting    up    secondary  injuries.     The  protoplasm  becomes 
disorganised  also  at  the  low  temperature. 

After  the  freezing  of  a  tissue  has  taken  place,  a  subse- 
quent rise  of  temperature  leads  to  a  process  of  thawing. 
This  in  many  cases  is  more  fatal  to  it  than  the  freezing,  but 
the  effect  depends  largely  on  the  rapidity  of  the  thawing. 
If  it  is  so  gradual  that  the  water  can  be  re-absorbed  into  the 
cells,  they  may  continue  to  live,  but  otherwise  the  organ  is 
'killed.  The  cells  become  flaccid  and  the  protoplasm  at 
once  ceases  to  have  the  power  of  maintaining  them  in  the 
turgid  condition. 

The  effect  of  the  absence  of  moisture  in  enabling  vegetable 
organisms  to  resist  cold  has  recently  been  examined  in  the 
case  of  seeds.  Several  kinds  of  these  have  been  found  to 
be  capable  of  germinating  after  immersion  for  several  hours 
in  liquid  hydrogen,  the  temperature  of  which  is  the  lowest 
at  present  known. 

A  similar  effect  is  found  at  the  other  end  of  the  scale. 
If  seeds  are  heated  very  gradually  some  will  withstand  a 
temperature  of  98°  C.  The  gradual  loss  of  water  is  a 
necessary  condition  for  this  immunity,  for  when  the  heat- 
ing is  conducted  so  quickly  that  the  water  is  not  driven  off 
at  a  low  or  moderate  temperature,  the  treatment  is  fatal 
in  all  cases.  Under  conditions  of  gradual  heating,  their 
temperature  being  maintained  at  60°  C.  for  twenty-four 
hours,  seeds  have  been  found  capable  of  germinating  after 
a  subsequent  exposure  to  98°  C.  lasting  for  ten  hours. 

Spores  of  bacteria  and  of  fungi  have  a  great  power  of 
resisting  high  temperatures,  and  this  is  probably  also  asso- 
ciated with  a  considerable  degree  of  dryness.  They  can 
withstand  boiling  in  water  for  some  time,  but  it  is  probable 
that  the  reason  why  they  are  not  destroyed  is  that  their 
walls  successfully  resist  the  passage  of  water  into  their 
interior. 

We  are  unable  at  present  to  explain  in  detail  the  causes 
of  the  death  of  protoplasm  under  the  conditions  of  extremes 
of  temperature.  We  can  only  say  that  under  these  con- 


TEMPEBATUKE  AND  ITS  CONDITIONS      335 

ditions  living  substance  ceases  to  carry  out  the  normal 
reactions  which  are  characteristic  of  it  so  long  as  it  is  what 
we  call  '  living,'  and  that  the  power  to  resume  them  after 
the  disappearance  of  the  adverse  conditions  is  not  regained 
by  it.  The  nature  of  life  and  the  intimate  causes  and 
features  of  death  are  still  beyond  our  knowledge. 


B36  VEGETABLE  PHYSIOLOGY 


CHAPTEK  XXI 

INFLUENCE    OF    THE    ENVIRONMENT    ON    PLANTS 

The  ultimate  form  of  a  plant  is  such  as  to  secure  the  most 
harmonious  relations  between  itself  and  its  environment. 
Such  relations  are  inseparable  from  a  healthy  condition. 
It  is  clear,  therefore,  that  with  varied  conditions  of  the  en- 
vironment we  must  expect  modifications  of  both  form  and 
structure.  It  is  impossible  in  such  a  work  as  the  present 
to  do  more  than  touch  upon  so  large  a  subject,  full  of  detail 
as  it  must  necessarily  be.  It  should  nevertheless  engage 
our  attention,  though  we  can  do  little  more  than  illustrate 
it,  for  it  has  a  very  important  bearing  upon  the  power  of 
a  plant  to  respond  to  variations  in  its  external  conditions,  a 
power  which  must  be  associated  with  a  kind  of  nervous 
system. 

According  to  the  nature  of  their  surroundings  and  the 
consequent  differences  in  their  mode  of  life,  we  find  in 
many  plants  certain  peculiarities  of  form  and  structure  in 
which  they  differ  from  most  of  those  which  we  have  hitherto 
considered.  Of  these  the  vascular  plants  which  live  in 
water  may  be  first  discussed,  as  the  direct  influence  of  the 
environment  is  most  conspicuous  in  their  case. 

These  aquatic  plants,  most  of  which  are  Spermophytes, 
but  which  include  a  few  of  the  Pteridophytes,  may  be 
divided  into  two  chief  groups  :  those  which  are  altogether 
submerged,  and  those  which  bear  floating  leaves  as  well  as, 
or  instead  of,  submerged  ones. 

In  the  former  case  the  plant-body  may  be  attached  by 
roots  to  the  bottom  of  the  stream  or  pool  in  which  it  lives, 


INFLUENCE  OF  ENVIRONMENT  ON  PLANTS  387 

or  it  may  be  altogether  floating.  The  stems  are  generally 
long  and  slender,  and  easily  swayed  to  and  fro  in  the  water. 
Some  have,  however,  very  short  stems  which  give  rise  to 
numerous  elongated  ribbon-like  leaves.  These  flexible 
stems  depend  for  their  support  upon  the  nature  of  the 
medium  in  which  they  live,  and  though  they  possess  a 


FIG.  139.— SECTION  OF  STEM  OF  Potamogeton,  SHOWING  AIR  PASSAGES 
IN  THE  CORTEX. 


certain  rigidity,  this  is  not  associated  with  any  great  de- 
velopment of  woody  tissue.  Generally  the  latter  is  reduced 
to  a  minimum ;  the  fibro-vascular  bundles  are  usually  few 
and  contain  few  lignified  elements.  Their  substance  is 
largely  parenchymatous,  and  the  cells  have  thin  walls. 
The  intercellular  space  system  is  often  very  complex,  large 
lacunae  filled  with  air  occupying  considerable  space  in  the 
distribution  of  the  tissues  (fig.  139).  Their  rigidity  is 

22 


338  VEGETABLE  PHYSIOLOGY 

secured  by  the  turgescence  of  the  parenchymatous  cells, 
and  buoyancy  is  much  assisted  by  the  air  in  the  lacunae. 

The  primary  root  is  generally  feebly  developed,  and,  as 
a  rule,  does  not  persist  through  the  life  of  the  plant.  The 
floating  forms  frequently  have  no  roots,  but  in  many  cases 
adventitious  roots  are  given  off  in  large  numbers  from  the 
various  nodes  of  the  stem.  The  root-hairs,  which  are  so 
characteristic  of  terrestrial  roots,  are  usually  either  very 
scanty  or  altogether  absent. 


FIG.  140. — SECTION  OF  LEAF  OF 
a,  lacunar  cavities ;   b,  vascular  bundle. 

The  epidermis  of  both  stem  and  root  is  not  cuticularised, 
and  therefore  the  cells  remain  capable  of  absorbing  the 
water  in  which  the  plant  is  living.  In  the  stem  this  tissue 
very  frequently  contains  chloroplasts. 

The  character  of  the  leaves  differs  according  to  the 
habitat.  Those  which  grow  in  rapid  streams  are  generally 
either  long  and  thin,  or  are  very  much,  and  finely,  divided, 
so  that  they  offer,  in  either  case,  no  resistance  to  the  force 
of  the  current.  In  more  sluggish  water  they  may  be  long 
and  ribbon-like,  but  are  frequently  broader,  and  sometimes 


INFLUENCE  OF  ENVIRONMENT  ON  PLANTS   339 

attain  a  considerable  size.  The  cell-walls  of  the  former 
are  often  thickened,  but  in  the  latter  the  tissue  is  always 
very  weak,  the  parenchyma  of  the  mesophyll  sometimes 
being  greatly  reduced.  In  Ouvirandra  as  the  leaf  becomes 
fully  developed  this  tissue  disappears,  only  the  veins  re- 
maining, so  that  it  presents  the  appearance  of  a  coarse 
grating  or  piece  of  lattice-work.  The  epidermis  of  a  sub- 


FIG.  141.— SECTION  OF  PETIOLE  OF  WATEK-LILY  (Nymphcea  alba). 
a,  c,  vascular  bundles ;   6,  d,  air-channels. 


merged  leaf  is  never  cuticularised,  and  it  contains  no  stomata. 
In  many  cases  large  lacuneB  are  formed  in  the  substance  of 
the  tissue,  particularly  when  the  lamina  is  somewhat  stout, 
as  in  Isoetes  (fig.  140). 

In  plants  with  floating  leaves  the  roots  and  stems  are 
similar  in  character  to  those  of  the  first  class.  The  leaves, 
however,  which  lie  upon  the  top  of  the  water,  are  usually 
tough  and  thick,  their  undersides  being  sometimes  deeply 

22* 


340  VEGETABLE  PHYSIOLOGY 

rugose.  They  have  not  the  much-divided  outline  character- 
istic of  submerged  leaves,  but  are  usually  simple  and  some- 
times of  considerable  size.  Those  of  the  Victoria  regia 
are  often  three  feet  in  diameter,  and  are  turned  up  at  the 
edges,  forming  a  rim,  which  helps  to  preserve  the  upper 
surface  from  being  wetted.  The  upper  epidermis  of  such 
floating  leaves  is  often  either  strongly  cuticularised,  or 
impregnated  with  a  waxy  secretion  serving  the  same  pur- 
pose. The  leaves  are  consequently  shiny  in  appearance, 
and  water  will  not  adhere  to  them.  These  floating  leaves 
bear  their  stomata  upon  the  upper  surface  only. 

The  petioles  are  long  and  flexible,  and  possess  a  peculiar 
power  of  adapting  themselves  to  varying  depths  of  water. 
Should  the  stream  in  which  they  live  become  shallow,  the 
leaves  still  remain  floating,  owing  to  the  power  of  the  petiole 
to  become  curved ;  should  the  water  rise,  the  petioles 
respond  by  resuming  their  growth,  so  as  always  to  keep 
pace  with  the  increased  depth.  Their  structure  resembles 
that  of  the  stem  in  that  they  are  composed  of  turgid  paren- 
chyma and  have  little  or  no  development  of  woody  tissue. 
They  also  contain  conspicuous  lacunae  or  air-channels 
(fig.' 141). 

Vegetative  reproduction  is  very  common,  branches 
becoming  detached  from  the  plant,  which  speedily  put  out 
adventitious  roots  of  their  own  and  form  new  plants. 

Their  watery  environment  explains  the  peculiarity  of 
their  structure.  From  the  nature  of  their  surroundings 
and  their  power  of  absorbing  liquid  through  their  epidermis 
we  can  easily  explain  the  absence  of  the  woody  tissue, 
which  we  have  seen  to  be,  when  present,  especially  devoted 
to  the  conducting  of  water  from  the  roots  throughout  the 
plant.  Their  absorbing  tissue  being  their  whole  superficial 
investment,  such  conduction  is  not  called  for,  for  nutritive 
purposes.  Their  transpiration,  moreover,  is  reduced  to  a 
minimum,  and  there  is  therefore  no  need  of  a  provision  for 
the  rapid  current  of  water  which  is  so  essential  to  the  well- 
being  of  a  terrestrial  plant,  in  which  this  function  is  so 


INFLUENCE  OF  ENVIRONMENT  ON  PLANTS  341 

prominent.  The  raw  materials  of  their  food  reach  them 
dissolved  in  the  water  in  which  they  live,  and  hence  they 
have  no  need  of  the  complicated  root  system  with  its 
absorbent  root-hairs,  which  is  so  characteristic  of  a  plant 
growing  in  ordinary  soil.  Gaseous  absorption  takes  place 
through  the  general  surface  to  a  large  extent,  but  this 
direct  supply  is  insufficient  for  respiration.  The  ordinary 


FIG.  142. — SECTION  OF  RHIZOME  OF  Marsilea. 
co.la.,  lacunoe  in  cortex. 

arrangements  for  aeration,  consisting  of  a  network  of  inter- 
cellular spaces  freely  in  communication  with  numerous 
stomata,  are  not  quite  the  same  in  plants  surrounded  by 
water.  We  have  seen  that  many  of  them  have  no  stomata, 
the  leaves  being  quite  submerged  ;  others  have  relatively 
few  on  the  upper  surfaces  of  the  floating  leaves.  The 
gaseous  interchange  between  the  interior  and  the  exterior 
is  consequently  greatly  impeded.  The  large  intercellular 


342  VEGETABLE  PHYSIOLOGY 

lacunae,  with  which  the  smaller  spaces  communicate,  form  a 
mechanism  by  which  this  difficulty  is  surmounted,  affording 
reservoirs  of  air  of  considerable  size  in  the  interior  of  all 
parts  that  are  submerged,  so  that  the  slow  rate  of  renewal  of 
air  does  not  impede  the  gaseous  interchanges  which  the 
protoplasts  require.  These  intercellular  reservoirs  are  not 
confined  to  the  vertical  stems,  petioles,  and  leaves,  but 
occur  also  in  the  more  woody  stems  or  rhizomes  which 
"many  of  these  plants  possess  (fig.  142). 

The  absence  of  the  transpiration  current  appears  to  be 
correlated  with  a  comparatively  small  development  of  the 
plant-body.  The  large  quantities  of  inorganic  salts  which 
the  dilute  solutions  absorbed  by  the  roots  carry  into  the 
plant,  in  cases  where  the  total  absorption  is  very  great 
owing  to  a  large  transpiration,  lead  to  a  large  increase  of 
constructive  activity.  In  the  absence  of  such  an  enormous 
absorption  the  plant-body  does  not  receive  the  materials 
necessary  for  the  acquirement  of  a  considerable  bulk. 
Aquatic  vascular  plants  are  consequently  never  very  large. 

The  difference  between  the  two  groups  of  aquatic  plants 
spoken  of  may  be  well  seen  in  such  forms  as  Cabomba, 
which  bears  both  submerged  and  floating  leaves.  These 
show  respectively  the  characteristics  described  in  each  case. 

Some  curious  adaptations  of  the  organism  to  its  environ- 
ments are  exhibited  by  certain  of  these  plants  which  live 
in  marshy  surroundings,  sometimes  being  nearly  or  wholly 
submerged,  and  at  others,  owing  to  the  drying  up  of  the 
water,  growing  upon  the  mud.  When  the  latter  fate  befalls 
them,  such  of  their  leaves  as  are  adapted  to  an  aquatic  life 
become  dried  up  and  perish.  The  upper  leaves,  which  have 
always  been  exposed  to  the  air,  do  not  suffer.  As  growth 
continues,  all  the  foliage  which  is  produced  is  of  the  terres- 
trial type.  On  the  other  hand,  when  the  plant-body  is 
submerged  the  new  leaves  are  all  of  the  aquatic  type. 
These  plants  are  often  spoken  of  as  amphibious. 

Some  aquatic  plants  are  saprophytic  in  their  mode  of 
life,  flourishing  best  in  water  which  is  contaminated  with 


INFLUENCE  OF  ENVIKONMENT  ON  PLANTS    343 

sewage  or  with  the  products  of  putrefaction.  They  are 
chiefly  certain  species  of  Algae  or  Fungi,  but  among  them 
may  be  included  a  few  Mosses  and  Phanerogams. 

Another  class  of  plants  which  show  a  definite  response 
in  their  structure  to  the  conditions  in  which  they  live  is 
that  to  which  the  term  XeropJiytes  has  been  applied.  These 
inhabit  different  situations,  all  of  which  are  characterised 
by  presenting  to  the  plant  a  very  small  supply  of  terrestrial 
water.  Many  grow  in  sandy  deserts,  exposed  to  great 
heat,  and  frequently  undergoing  long  periods  of  drought. 
Others  grow  upon  a  rocky  substratum,  and  their  roots  are 


FIG.  143. — LEAF  OF  Saxifraga  incrustata,  SHOWING  ABSORBING  ORGAN.     X  20. 

confined  to  the  crannies  and  crevices  which  are  present 
in  the  rock.  Others  are  found  in  more  temperate  countries, 
occupying  light  sandy  soils  which  cannot  retain  any  con- 
siderable quantity  of  water.  Such  xerophytic  plants  as 
are  woody  in  habit  frequently  show  considerable  tendency 
to  diminish  their  leaf-surface,  probably  to  reduce  evapora- 
tion and  conserve  their  stock  of  water.  They  often  have 
many  of  their  branches  transformed  into  thorns  or  spines, 
and  very  frequently  their  leaves  show  similar  reduction. 
Others  which  contain  little  wood  are  succulent,  and  their 
surfaces  are  covered  by  a  very  thick  and  tough  epidermis, 
which  is  strongly  cuticularised.  Many  of  those  which  grow 


344  VEGETABLE  PHYSIOLOGY 

upon  rocks  have  leaves  which  show  special  structures  for 
absorbing  water  from  rain  or  dew.  Several  plants,  among 
which  some  species  of  Saxifrage  are  conspicuous,  possess 
a  number  of  glandular  structures  upon  the  teeth  of  their 
thick  narrow  leaves.  Each  consists  of  a  small  mass  of 
cells  with  delicate  walls,  which  lie  immediately  under  the 
epidermis  of  a  small  depression  of  the  surface,  and  which 
communicate  with  the  exterior  by  a  few  fine  pores  which 
perforate  the  latter.  The  epidermis  of  this  depression  is 
made  up  of  cells  with  thin  non-cuticularised  walls.  Each 
so-called  gland  is  in  contact  with  the  end  of  a  fibro-vascular 
bundle,  whose  sheath  is  carried  forward  over  the  general 
mass  of  delicate  cells  (fig.  143).  The  depression  of  the 
surface  is  filled  with  a  mass  of  carbonate  of  lime,  which 
is  originally  excreted  by  the  leaf,  and  which  is  held  in  its 
place  by  a  few  papillae  which  project  from  the  epidermis. 
Such  an  arrangement  serves  a  double  purpose  ;  any  dew 
or  rain  which  reaches  the  surface  of  the  leaf  is  absorbed  by 
the  carbonate  of  lime,  and  can  make  its  way  slowly  into  the 
gland,  whence  it  passes  into  the  fibro-vascular  system ; 
while,  when  the  leaf  is  dry,  the  incrusting  mineral  matter 
serves  as  a  plug  to  the  depression,  and  reduces  transpiration. 

Many  plants  which  inhabit  sandy  deserts  possess  similar 
mechanisms ;  some  excrete  carbonate  of  lime,  others 
crystalline  accumulations  of  common  salt.  The  latter  can 
not  only  absorb  dew  and  rain,  but  can  also  condense  and 
take  up  moisture  from  the  air.  They  are  found  occurring  in 
such  sandy  wastes  as  are  by  the  seashore  or  near  salt  lakes. 

Many  trees  which  grow  in  temperate  climates,  in  poor 
sandy  soil  on  the  margin  of  streams,  show  a  somewhat 
similar  mechanism,  but  the  excretion  from  their  leaves  takes 
the  form  of  a  kind  of  resinous  varnish  or  balsam  which  can 
be  readily  wetted,  and  which  can  absorb  water.  In  some 
cases  so-called  glandular  hairs  discharge  a  similar  function. 

The  water  which  is  absorbed  in  this  way  is  rarely  pure, 
but  contains  traces  of  sulphuric  acid  and  ammonia,  which, 
though  trifling  in  amount,  are  no  doubt  of  value  in  the 


INFLUENCE  OF  ENVIRONMENT  ON  PLANTS    345 

nutritive  processes.  The  adaptation  to  their  environment 
which  these  plants  exhibit  is  thus  chiefly  in  the  direction 
of  economising  a  limited  water  supply. 

The  influence  of  the  environment  on  the  form  of  the 
plant  can  be  seen  equally  well  in  the  case  of  such  plants 
as  grow  in  Alpine  regions,  where  the  cold  is  usually  intense, 
and  the  atmosphere  for  long  periods  so  humid  that  trans- 
piration is  only  occasionally  possible,  and  where  consequently 
the  absorption  of  the  raw  materials  of  the  food  is  much 
impeded.  Similar  conditions  mark  the  bleak  moorlands  of 


FIG.  144. — TRANSVERSE  SECTION  OF  ROLLED  LEAF  OF  HEATH,    st.  STOMATA  IN  THE 

GROOVE. 

temperate  climates.  These  show  very  great  differences 
between  the  extremes  of  temperature  which  mark  summer 
and  winter  respectively.  The  water  supply  also  shows  very 
great  variations  at  different  times  of  the  year.  The  plants 
are  generally  of  comparatively  small  size,  and  bear  thick, 
often  rolled-up,  leaves  which  are  evergreen.  The  thick 
exterior  and  the  general  hardness  of  the  leaf  are  a  response 
to,  and  a  defence  against,  the  cold.  In  the  heaths,  which 
may  be  regarded  as  typical  moorland  plants,  transpiration 
is  reduced  to  a  minimum,  large  air-chambers  in  the  leaf 
with  only  a  few  stomata,  and  those  situated  in  a  deep  groove, 
providing  for  the  aeration  of  the  protoplasts.  During  the 


346  VEGETABLE  PHYSIOLOGY 

cold  the  closing  of  these  almost  hidden  stomata  guards  the 
plant  from  the  evaporation,  which,  if  unchecked,  would 
lead  to  a  loss  of  heat  that  might  be  fatal  to  it.  The  meta- 
bolism being  reduced  by  the  low  temperature,  the  contents 
of  the  air  reservoirs  suffice  for  such  interchanges  of  gases  as 
are  imperative,  and  for  the  coincident  exhalation  of  watery 
vapour  by  the  protoplasts,  but  as  these  contents  are  very 
slowly  renewed  the  total  evaporation  is  but  slight.  When, 
on  the  other  hand,  for  a  part  of  the  year  the  temperature 
is  high,  the  spacious  reservoirs  provide  for  a  very  rapid 
transpiration  as  soon  as  the  stomata  are  open,  a  very 
large  spongy  mesophyll  abutting  on  them  (fig.  144).  The 
evergreen  leaves  also  are  an  expression  of  the  struggle 
against  the  difficulty  of  the  absorption  of  food  materials, 
which  in  such  atmospheric  conditions  is  possible  for  only 
a  limited  period  of  the  year.  By  preserving  its  leaves  green 
the  plant  can  take  advantage  riot  only  of  the  light  of  summer, 
but  also  of  those  bright  sunny  days  which  occur  occasionally 
during  the  cold  season,  and  thus  improve  every  opportunity 
afforded  it. 

Some  lowland  plants  show  a  similar  response  to  their 
environment,  the  form  and  structure  of  different  individuals 
of  the  same  species  varying  to  a  certain  extent,  according 
to  their  advantages  or  the  reverse,  under  such  conditions 
as  sunlight  or  shade,  drought  or  moisture,  exposure  to  or 
protection  from  cold  winds,  &c. 

Epiphytic  plants  show  some  conspicuous  modifications 
of  their  structure  in  consequence  of  their  peculiar  habit  of 
life.  They  usually  live  upon  the  surfaces  of  trees,  to  which 
they  cling  by  various  means,  but  from  which  they  derive 
no  nourishment  except  such  as  is  afforded  by  accumulations 
of  debris,  &c.,  upon  the  trunks.  They  are  not  parasitic, 
but  merely  live  upon  the  tree  as  other  plants  grow  upon 
rocks  or  cliffs.  Mosses  and  Liverworts  are  very  largely 
epiphytic,  as  are  certain  species  of  Phanerogams ;  the 
latter  are  very  specialised  forms,  and  show  most  adapta- 
tion of  form  and  structure.  Perhaps  the  most  remarkable 


INFLUENCE  OF  ENVIRONMENT  ON  PLANTS    347 

feature  about  them  is  their  aerial  adventitious  roots,  which 
are  given  off  in  some  cases  from  every  node  of  the  stem, 
so  that  each  internode  has  its  own  supply.  These  are 
often  long  cord-like  structures,  which  are  of  some  thickness, 
often  contain  chloroplasts,  and  are  either  covered  by  a 
special  epidermal  development,  or  give  rise  to  dense  masses 
of  root-hairs.  In  the  first  case,  which  is  common  among 
epiphytic  orchids,  the  epidermis  is  many  cells  thick,  and 
is  known  as  the  velamen.  The  cells  are  small  trache'ids, 
with  curious  reticulated  or  spiral  thickenings,  and  are  often 
perforated.  These  peculiar  trache'ids  contain  only  air,  and 
the  velamen  has  consequently  a  curious  glistening  greenish 
appearance.  The  mass  of  trache'ids  forms  a  kind  of  spongy 
covering  to  the  root,  and  is  capable  of  condensing  and 
absorbing  aqueous  vapour  from  the  moist  atmosphere  which 
usually  surrounds  it.  At  other  times  when  the  air  is  dry  and 
there  is  ar  danger  of  evaporation  from  the  root,  this  valamen 
acts  as  a  protective  membrane  against  loss  of  water  in  this 
way.  The  second  case  is  illustrated  by  many  aroids,  and 
the  dense  plexus  of  root-hairs  borne  upon  the  aerial  roots 
serves  the  same  purpose  as  the  velamen  of  the  orchids. 
Besides  these  roots,  thus  adapted  to  absorb  watery  vapour 
from  the  air,  epiphytes  frequently  have  others  which  are 
closely  applied  to  the  surface  of  the  bark  on  which  they 
are  growing.  These  are  often  strap-shaped,  and  cling  very 
closely  to  the  tree,  absorbing  from  the  bark  the  soluble 
products  of  its  decomposition  and  any  mineral  debris  that 
may  be  accidentally  carried  thither.  The  small  amount 
of  such  food  stuffs  available  will  explain  the  relatively  large 
development  of  the  root  system,  which  is  in  much  greater 
proportion  than  in  ordinary  terrestrial  plants. 

Parasites  are  another  class  of  plants  that  have  under- 
gone much  modification  of  structure  in  consequence  of  their 
mode  of  life.  The  parasitic  habit  is  seen  most  completely 
in  the  group  of  Fungi,  but  it  is  by  no  means  confined  to 
them.  We  find  many,  cases  of  partial  or  complete  para- 
sitism among  flowering  plants.  In  all  cases  we  notice  that 


348  VEGETABLE  PHYSIOLOGY 

the  parasitic  habit  is  associated  with  a  degeneration  of 
structure,  which  especially  affects  the  vegetative  organs. 

The  fungus  which  is  parasitic  in  habit  derives  all  its 
nourishment  from  the  plant  or  animal  whose  tissues  it  has 
invaded.  Other  plants  of  the  same  group  are  not  parasitic, 
but  live  upon  decomposing  organic  matter,  being  known 
as  saprophytes.  Their  mode  of  nutrition  is,  however, 
essentially  the  same.  They  have  all  lost  the  chlorophyll 
apparatus  characteristic  of  the  green  plant,  and  cannot 


FIG.  146. — Thesium  alpinum,  SHOWING  THE  SUCKERS  ox  THE  ROOTS. 
(After  Kerner.) 


therefore  work  up  the  food  materials  that  the  latter  absorbs 
from  the  air.  Instead,  therefore,  of  absorbing  carbon 
dioxide,  these  plants  take  in  their  carbohydrate  food  ready 
made  in  the  form  of  an  organic  compound  of  some  complexity, 
which  is  usually  some  kind  of  sugar.  Saprophytes  can 
absorb  nitrogen  in  the  same  combinations  as  a  green  plant, 
but  they  appear  to  utilise  compounds  of  ammonia  in  pre- 
ference to  nitrates.  No  doubt  their  protoplasm  is  ultimately 
fed  with  the  same  substances  as  is  that  of  the  higher  plants, 


INFLUENCE  OF  ENVIRONMENT  ON  PLANTS    349 

but  they  lack  a  great  deal  of  the  constructive  power  of  the 
latter. 

The  degradation  of  the  structure  of  such  plants  is 
associated  with  the  absence  of  the  constructive  processes 
which  depend  on  the  presence  of  chlorophyll.  Their  body 
is  usually  composed  chiefly  of  delicate  hyphae,  which  ramify 
in  the  nutrient  substratum,  either  living  ur  dead,  and  which 
absorb  elaborated  products  of  some  complexity  freely  by 
their  whole  surface.  They  have,  therefore,  no  need  of 


FIG.  146. — Thesium  alpinum.     PIECE  OF  A  ROOT  WITH  SUCKER 
IN  SECTION,     x   35.     (After  Kerner.) 


differentiated  absorbing  or  conducting  tissues,  and  these  are 
consequently  not  developed.  A  further  consequence  of  the 
ease  with  which  they  obtain  their  food  is  the  readiness  with 
which  vegetative  and  asexual  reproduction  is  brought  about ; 
hence  sexuality  is  in  many  cases  non-existent  among  them. 
Phanerogams  which  are  completely  parasitic  show  a 
similar  degradation  of  structure.  They  possess  no  chloro- 
plasts,  their  leaves  are  absent  or  reduced  to  the  condition 
of  scales,  while  their  stems  are  often  thick  and  succulent. 
Their  roots  are  replaced  by  the  so-called  haustoria,  which 


350  VEGETABLE  PHYSIOLOGY 

penetrate  into  the  tissues  of  their  hosts,  complete  fusion  of 
the  tissue  of  the  host  and  the  parasite  frequently  taking 
place.  We  have  representatives  of  such  parasites  in  the 
British  flora  in  Cuscuta  and  the  Orobanchacece. 

Many  of  the  plants  belonging  to  the  Santalacece  and 
the  Scrophulariacece  show  a  partial  parasitism  of  this  kind. 
They  have  short  stems  which  bear  green  functional  leaves, 
but  are  peculiar  in  that  their  roots  become  attached  by 
curious  sucker-like  bodies  to  the  roots  of  other  plants 
growing  near  them  (figs.  145,  146),  and  from  these  suckers 
absorbing  cells  are  developed  which  penetrate  into  the 
substance  of  their  hosts  and  draw  nourishment  from  them. 
They  are  generally  described  as  root  parasites.  The  Mistletoe 
behaves  similarly,  striking  its  haustoria  into  the  tissue 
of  the  branches  of  the  apple,  oak,  poplar,  &c.  The  para- 
sitism is  partly  compensated  by  the  fact  that  its  leaves 
remain  green  when  the  host  has  lost  its  foliage,  and  by 
their  activity  they  to  some  extent  assist  the  tree  on  which 
the  mistletoe  is  growing.  The  relationship  seems  to  be 
almost  one  of  symbiosis  rather  than  of  parasitism.  Probably 
the  relationship  of  the  root-parasites  and  their  hosts  is 
also  one  of  mutual  assistance  rather  than  true  parasitism. 

The  habit  of  capturing  insects,  which  we  have  seen  to  be 
characteristic  of  several  plants  of  very  different  forms,  may 
also  be  looked  upon  as  connected  with  their  environment. 
Many  of  them,  e.g.  Drosera,  grow  upon  a  substratum  which 
is  largely  composed  of  plants  of  Sphagnum,  and  which  yields 
to  them  a  very  limited  supply  of  nitrogenous  compounds  ; 
others  are  found  growing  on  the  surface  of  rocky  mountains, 
into  the  chinks  of  the  stones  of  which  their  roots  penetrate  ; 
others  again  flourish  in  the  sandy  soil  of  deserts  ;  in  all  of 
which  situations  compounds  of  nitrogen  exist  only  in  very 
small  amount.  The  organic  substances  yielded  by  the 
decomposing  bodies  of  the  captured  insects  must  therefore 
form  a  valuable  supplement  to  the  ordinary  sources  of 
•nitrogen. 

These  illustrations  of  the  modification  of  structure  and 


INFLUENCE  OF  ENVIRONMENT  ON  PLANTS    351 

general  habit  serve  to  show  us  that  there  is,  throughout 
the  vegetable  kingdom,  a  constant  effort  on  the  part  of  the 
plant  to  adapt  itself  to  its  surroundings,  so  as  to  make  the 
best  of  the  external  conditions.  This  struggle,  though 
perhaps  most  easily  realised  by  a  survey  of  large  groups 
which  are  affected,  is  really  carried  out  by  the  individual 
organisms,  and  the  comparatively  striking  effects  seen  are 
the  result  of  the  cumulative  efforts  of  a  long  series  of  indi- 
viduals, each  of  whom  has  possessed  in  different  degrees 
powers  of  reacting  to  varying  external  conditions.  These 
powers  will  be  considered  in  subsequent  chapters. 


352  VEGETABLE  PHYSIOLOGY 


CHAPTER  XXII 

THE    PROPERTIES    OF   VEGETABLE    PROTOPLASM 

The  influence  of  the  environment  upon  the  structure  of 
plants  we  have  seen  to  be  far-reaching.  Different  con- 
ditions of  the  surroundings  are  followed  by  differences  of 
structure,  which  are  greater  in  proportion  as  the  time 
during  which  those  conditions  act  is  more  and  more  pro- 
longed. The  living  substance  of  the  plant  is  clearly  the  part 
influenced  by  the  environment,  for  we  have  seen  that  the 
skeleton  and  other  non-living  parts  of  the  plant  owe 
their  construction  to  its  activity.  We  may  therefore  with 
advantage  pause  at  this  point  to  examine  a  little  more 
closely  the  properties  which  are  exhibited  by  vegetable 
protoplasm. 

We  have  seen  throughout  all  the  foregoing  chapters 
that  all  the  processes  which  conduce  to  the  well-being  of 
the  plant  are,  to  a  large  extent,  if  not  entirely,  under  the 
control  of  the  living  substance.  Though  the  absorption 
of  the  raw  materials  of  its  food  from  the  air  and  the  soil  is 
due  to  physical  processes,  these  are  nevertheless  largely 
regulated  by  the  behaviour  of  the  protoplasm  under  all  sorts 
of  varying  conditions.  The  manufacture  of  food  from  these 
crude  materials,  and  its  subsequent  distribution,  the  accumu- 
lation and  dissipation  of  energy,  the  processes  of  nutrition 
and  growth,  are  all  subject  to  the  same  regulation. 

But  there  are  also  other  properties  of  protoplasm  which 
have  not  so  far  been  more  than  incidentally  referred  to. 
The  plant  exhibits  particularly  the  power  of  appreciating 


PKOPEKTIES  OF  VEGETABLE  PEOTOPLASM  358 

changes  in  its  surroundings,  and  is  capable  of  adapt- 
ing itself  in  various  ways  to  such  changed  conditions. 
In  many  cases  the  adaptation  in  question  takes  the  form 
of  a  spontaneous  movement,  in  which  the  living  substance 
is  concerned  in  a  manner  which  seems  to  resemble  the 
behaviour  of  animal  protoplasm.  In  others  the  response 
to  such  changes  presents  itself  to  us  as  a  modification  of 
the  normal  behaviour  of  the  living  substance  with  regard 
to  the  vital  processes  we  have  examined,  and  in  particular 
to  the  entry  of  water  into  the  vacuoles  of  the  cells  or  its 
transmission  outwards. 

When  we  examine  the  phenomena  of  movement  we 
find  that  though  evidence  of  contractility  is  procurable, 
this  phenomenon  is  of  somewhat  rare  occurrence  in  plants. 
Certain  plants  at  particular  times 
emit  from  their  body  small  masses  of 
naked  protoplasm  which  are  furnished 
with  a  varying  number  of  long  fila- 
ments (fig.  147).  These  filaments, 
which  are  protoplasmic  also,  are 
ordinarily  in  a  state  of  active  vibra- 
tion, causing  currents  in  the  water  Fia-  147.  —  ZOOSPORB  OF 

'  fe  .  Ulothrix.      x   500. 

in  which  they  live,  which  float  them 
quickly  from  place  to  place.  Among  these  free-swimming 
protoplasts  may  be  mentioned  the  zoospores  of  the  Algae 
and  Fungi,  and  the  antherozoids  of  these  and  higher  plants. 
The  movement  is  a  spontaneous  one,  the  organisms  being 
endowed  with  the  property  of  locomotion,  which  they  exer- 
cise in  the  discharge  of  their  ordinary  life-work.  Though 
put  forth  in  the  absence  of  any  external  stimulation,  the 
protoplasts  are  capable  of  receiving  such  impulses  and 
modifying  the  vibratile  action  accordingly. 

The  mechanism  of  the  movement  is  probably  the  con- 
traction of  each  side  of  the  filament  or  cilium  alternately, 
or  of  the  part  of  the  cell  just  at  the  point  of  attachment- 
The  impulse  leading  to  the  movement  must  be  sought  in 
some  decomposition  originating  in  the  protoplasm  itself, 

23 


354 


VEGETABLE  PHYSIOLOGY 


and  not  excited  by  any  stimulation  from  without.     The 
phenomenon  is  often  spoken  of  as  ciliary  motion. 

Of  a  somewhat  similar  character  is  the  curious  creeping 
movement  of  the  Myxomycetous  Fungi.  In  a  few  cases  the 
zoospores  of  these  organisms  are  furnished  with  cilia  or 
flagella,  resembling  those  of  the  zoospores  already  men- 
tioned, but  more  frequently  each  consists  of  a  minute  mass 
of  naked  protoplasm,  which  makes  its  way  over  the 
surface  of  its  substratum  by  putting  out  blunt  processes 
of  its  own  substance,  known  as  pseudopodia  (fig.  148). 


FIG.  148. — STAGES  IN  CONSTRUCTION  OF  THE  PLASMODIUM  OF  A 
Myxomycete, 


After  a  while  a  number  of  these  zoospores  become  fused 
together  to  form  a  large  jelly-like  mass,  known  as  a 
plasmodium.  This  colony  of  protoplasts  then  makes  its 
way  slowly  over  its  substratum  by  similar  pseudopodial 
movements.  Each  pseudopodium  is  a  protrusion  of  the 
ectoplasm,  and  the  more  fluid  endoplasm  is  in  some  way 
drawn  into  the  different  protrusions,  so  that  the  rest  of 
the  cell  or  of  the  plasmodium  follows  the  extension  of  the 
pseudopodium  and  is  dragged  after  it.  Which  part  of 
the  operation  corresponds  to  the  act  of  contraction  is 
disputed,  but  it  seems  probable  that  it  is  the  second,  and 
that  the  first  protrusion  is  of  the  nature  rather  of  relaxa- 
tion. The  movement,  like  that  of  ciliary  action,  is  a 


PROPEBTIES  OF  VEGETABLE  PKOTOPLASM  355 

property  of  the  organism,  and  is  used  by  it  in  the  ordinary 
course  of  its  life,  even  in  the  absence  of  stimulation. 

Among  the  lowliest  of  the  Algae  or  seaweeds  some  other 
organisms  are  conspicuous  by  their  power  of  locomotion. 
These  are  the  Diatoms  which  are  so  prominent  in  ponds 
and  sluggish  streams.  They  are  unicellular  plants  of  very 
minute  size,  each  of  which  consists  of  a  protoplast  encased 
in  two  silicified  shells  or  valves  which  fit  together  very 
tightly,  one  overlapping  the  other  by  its  edges.  The  cell- 
wall  which  forms  each  valve  is  strongly  impregnated  with 
silicia,  the  latter  being  deposited  in  patterns  which  are 
often  of  great  regularity  and  beauty.  The  plants  are  not 
provided  with  cilia,  nor,  so  far  as  we  know,  are  the  silicious 
valves  perforated  in  any  way.  Each  diatom  is,  however, 
capable  of  effecting  a  peculiar  gliding  and  very  rapid  move- 
ment through  the  water,  the  mechanism  of  which  is  at 
present  not  clearly  understood. 

Certain  filamentous  Algse,  known  as  the  Oscillatorice, 
also  carry  out  a  peculiar  movement.  They  consist  of  long 
chains  of  protoplasts,  each  separated  from  its  neighbour 
by  a  cell-wall,  and  the  whole  thread  surrounded  or  coated 
by  a  peculiar  semi-gelatinous  sheath.  Each  chain  is 
anchored  to  a  substratum  of  stone  or  rock  at  one  end,  and 
the  free  portion  is  in  constant  waving  or  twisting  motion 
to  and  fro,  a  movement  which  is  quite  independent  of 
currents  in  the  water,  being  exhibited  in  the  total  absence 
of  such  disturbance.  The  movement  appears  to  resemble 
that  of  the  Diatoms,  but  its  mechanism  is  at  present  un- 
explained. Like  the  others  so  far  discussed,  it  is  one  of 
the  features  of  the  life  of  the  organisms,  and  is  carried  out 
by  their  protoplasm  without  excitation  by  an  external 
stimulus. 

In  certain  organisms  of  still  humbler  type  another  mani- 
festation of  the  power  of  contractility  can  be  observed. 
These  are  unicellular  beings  consisting  of  small  unclothed 
masses  of  protoplasm.  In  their  substance  at  some  point 
there  may  be  seen  a  clear  space  or  vacuole  which  exhibits 

23* 


356  VEGETABLE  PHYSIOLOGY 

a  more  or  less  regular  pulsation,  assuming  slowly  the 
appearance  of  a  nearly  spherical  cavity  and  then 
suddenly  disappearing,  recalling  the  active  contraction 
of  animal  protoplasm.  These  pulsating  or  contractile 
vacuoles  can  be  seen  very  well  in  Chlamydomonas. 

The  power  of  movement  which  is  thus  exhibited  by 
many  of  the  lowlier  plants  may  be  distinguished,  however, 
from  certain  of  the  movements  of  portions  of  higher  plants 
which  have  already  been  alluded  to,  and  which  will  be 
discussed  more  fully  subsequently.  These  movements 
include  the  circumnutation  of  growing  organs,  the  closing 
of  the  leaves  of  Dioncea,  the  bending  of  the  tentacles  of 
Drosera,  and  many  others.  These  are  brought  about  in 
multicellular  organs,  and  by  a  mechanism  different  from  the 
one  now  under  discussion,  the  movement  being  secondary 
and  following  indirectly  on  a  change  in  the  behaviour  of 
the  protoplasm  of  certain  of  the  cells,  which,  instead  of  con- 
tracting, modifies  its  resistance  to  the  escape  of  the  water 
which  they  contain.  In  one  or  two  cases,  as  in  the  curving 
of  certain  tendrils  and  in  the  drooping  of  certain  leaves  in 
response  to  stimulation,  the  hydrostatic  disturbance  seems 
to  be  attended  by,  and  perhaps  partly  dependent  upon,  a 
contraction  of  the  protoplasm  of  certain  cells.  These 
phenomena  will  be  discussed  in  a  subsequent  chapter, 
and  need  only  be  alluded  to  here  as  possibly  showing  the 
inherent  power  of  contractility  residing  in  the  protoplasm. 

Though  the  power  of  locomotion,  which  we  have  seen 
in  many  cases  to  exist,  is  an  evidence  of  certain  powers  of 
movement  or  contractility  possessed  by  living  substance, 
it  must  not  be  inferred  that  only  organisms  which  are  free 
to  move  are  possessed  of  these  or  similar  properties.  Loco- 
motion is  impossible  to  the  great  majority  of  plants  on 
account  of  their  relationship  to  their  environment.  There 
is,  however,  a  certain  amount  of  evidence  to  show  that  the 
instability  which,  in  the  cases  discussed,  finds  its  expression 
in  movement,  is  a  property  of  living  substance  in  general. 
We  find  many  cases  in  which  movement  of  the  living 


PROPERTIES  OF  VEGETABLE  PROTOPLASM  357 

substance  can  be  observed  in  the  interior  of  ordinary  cells. 
It  can  only  be  seen  when  the  protoplasm  is  more  or  less 
filled  with  granules,  as  in  their  absence  it  is  so  transparent 
that  it  is  impossible  to  say  whether  it  is  in  motion  or  not. 
In  the  leaf  of  Elodea  we  find  a  very  good  instance  of  this 
movement.  Each  cell  contains  a  considerable  quantity  of 
water,  so  that  the  protoplasm  for  the  main  part  is  found  as 
a  layer  lining  the  cell-wall.  This  layer  consists  of  two  parts, 


FIG.  149.— CELLS  FROM  THE  LEAF 
OF  Elodea.     X   500. 

n,  nucleus ;  p,  protoplasm,  in  which 
arc  embedded  numerous  chloroplasts. 
The  arrows  show  the  direction  of  the 
movement  of  the  protoplasm. 


FIG.  160. — Two  CELLS  FROM  A 
STAMINAL  HAIR  OF  Trades- 
cantia.  X  300. 

The  arrows  show  the  direction 
of  the  movement  of  the  pro- 
toplasm. 


the  inner  one  of  which  contains  large  numbers  of  chloroplasts. 
It  is  this  layer  which  exhibits  the  movement,  which  can  be 
seen  as  a  streaming  motion  of  the  plastids,  the  whole 
layer  flowing  slowly  round  the  cell  (fig.  149). 

In  other  cases,  particularly  in  long  pollen-tubes,  where 
the  distribution  of  the  protoplasm  is  so  far  different  that 
bands  or  bridles  of  it  cross  the  vacuole  in  various  directions, 
the  movement  has  a  more  complicated  course,  streams  of 
granules  passing  along  these  bridles  as  well  as  along  the 


358  VEGETABLE  PHYSIOLOGY 

peripheral  portions  of  the  protoplasm.  These  two  cases  of 
streaming  movements  of  protoplasm  are  spoken  of  as  rotation 
and  circulation  respectively.  There  is  no  difference  appa- 
rently between  them,  except  what  is  involved  in  the  different 
distribution  of  the  protoplasm  in  the  cells.  Other  instances 
are  met  with  in  the  staminal  hairs  of  Tradescantia  (fig.  150), 
the  leaves  of  Vallisneria,  the  internodal  cells  of  Chara  and 
Nitella,  and  the  unicellular  Desmids. 

It  is  evident  from  the  structure  of  most  vegetable  organisms 
that  the  possession  of  a  power  of  active  contractility, 
such  as  is  possessed  by  most  animals,  would  be  of  com- 
paratively little  use  to  them.  Though  flexible  to  a  certain 
extent,  they  are  possessed  of  a  fair  amount  of  rigidity, 
which  under  ordinary  conditions  they  do  not  relax.  We 
have  seen  that  one  of  the  most  important  relations  of 
their  life  is  that  which  is  maintained  between  the  proto- 
plasm and  water.  Each  cell  or  protoplast  is  so  organised 
as  to  contain  its  own  appropriate  store,  upon  the  posses- 
sion and  renewal  of  which  its  efficiency  as  a  member  of 
the  colony,  if  not  its  actual  life,  depends.  The  regulation 
of  this  supply  of  water  is  of  the  first  importance  to  the 
plant,  and  it  is  not  surprising,  therefore,  to  find  that  such 
a  regulatory  power  is  one  of  the  properties  of  vegetable 
protoplasm. 

All  healthy  vegetable  cells  are  during  life  in  a  condi- 
tion which  is  known  as  turgor.  The  cell  is  overfull  of  water, 
so  that  a  certain  internal  hydrostatic  pressure  is  exerted 
on  the  whole  surface  of  the  limiting  membrane,  which 
is  stretched  accordingly.  As  the  membrane  possesses 
elasticity,  the  wall  in  turn  exerts  a  pressure  upon  the  fluid 
inside  it,  and  during  healthy  life  a  certain  equilibrium 
exists  between  these  two  pressures.  Such  a  cell  is  called 
turgid,  and  the  degree  of  its  distension  is  the  measure  of 
its  turgidity.  This  turgor  can  vary  within  fairly  wide 
limits,  consistently  with  the  health  of  the  cell.  The  turgor 
depends  chiefly  upon  two  factors,  both  of  which  are  capable 
of  control.  The  water  is  caused  to  enter  the  cell,  as  we 


PROPERTIES  OF  VEGETABLE  PROTOPLASM  859 

have  seen,  by  the  formation  of  various  osmotically 
active  substances  in  its  interior,  which  have  an  attraction 
for  water,  the  quantity  which  enters  depending  upon  the 
nature  and  amount  of  such  substances  present.  We  have 
seen  already  that  the  regulation  of  osmotic  material  is 
controlled  by  the  protoplasm.  But  besides  this  another 
important  factor  exists  in  the  greater  or  less  difficulty  with 
which  water  is  enabled  to  pass  through  the  protoplasmic 
membranes.  The  power  of  altering  its  permeability  by 
water  and  so  varying  the  distension  of  its  elastic  membranes 
is  a  property  of  protoplasm  which  is  of  the  highest  import- 
ance in  the  mechanics  of  the  cell.  It  takes  the  place, 
practically,  which  is  held  by  the  power  of  contractility  in 
the  living  substance  of  animals.  No  doubt  it  can  be  called 
into  play  during  life  under  constant  conditions,  but  it 
becomes  much  more  marked  when  the  plant  is  subjected  to 
particular  kinds  of  stimulation.  A  ready  instance  of  its 
employment  under  the  former  conditions  is  afforded  by 
the  variations  of  turgidity  and  subsequent  growth  which 
we  have  already  spoken  of  as  inducing  circumnutation 
(p.  321).  Instances  of  its  following  upon  stimulation  will 
be  discussed  more  appropriately  in  a  later  chapter. 

The  facts  thus  briefly  narrated  impress  upon  us  the 
belief  that  all  protoplasm  is  the  seat  of  active  molecular 
movement,  the  intensity  or  vigour  of  which,  as  well  as  the 
forms  of  its  manifestations,  varies  very  greatly  in  different 
cases.  Indeed,  the  life  of  the  protoplasm  is  intimately 
bound  up  with  such  a  motile  condition.  The  manifesta- 
tions are  in  all  cases  appropriate  to  the  manner  of  life  and 
the  surroundings  of  the  organism  under  observation  ;  they 
may  take  the  form  of  locomotion,  of  contractility,  or  of 
variation  of  permeability,  leading  to  the  regulation  of 
turgescence. 

If  we  look  back  to  the  behaviour  of  the  contractile 
vacuole  of  Chlamydomonas,  we  are  struck  by  the  fact  that 
its  pulsations  occur  with  a  certain  definite  intermittence  so 
long  as  they  are  not  interfered  with  by  external  conditions. 


860  VEGETABLE  PHYSIOLOGY 

The  vacuole  dilates  slowly,  reaches  a  certain  size,  and 
suddenly  disappears  ;  then  is  gradually  formed  again,  and 
the  series  of  events  is  repeated.  This  regular  intermittence 
constitutes  what  is  often  spoken  of  as  rhythm. 

The  rhythm  which  is  so  easily  seen  in  the  case  of  pul- 
sating vacuoles  is  characteristic  also  of  those  less  obvious 
changes  in  protoplasmic  motility  which  lead  to  the  varia- 
tions of  turgidity  in  different  organs,  particularly  in  those 
which  are  growing.  We  have  already  seen  that  during 
the  growth  in  length  of  a  symmetrical  organ,  such  as  a 
stem  or  root,  the  apex  points  successively  to  all  points  of 
the  compass,  the  successive  changes  of  position  being 
spoken  of  as  circumnutation.  This  is  the  result  of  a 
rhythmic  variation  of  the  turgidity  of  the  cells  of  the 
cortex.  If  we  consider  a  longitudinal  band  of  such  cells,  we 
find  that  at  a  certain  moment  the  cells  are  at  their  point  of 
maximum  turgidity,  and  the  growing  apex  is  made  to  bend 
over  in  a  direction  diametrically  opposite  to  this  band. 
The  turgidity  of  this  band  then  gradually  declines  to  a 
minimum,  and  again  increases  slowly  to  a  maximum.  If 
we  conceive  of  the  circumference  of  the  organ  as  divided 
into  a  number  of  such  bands,  we  can  gain  an  idea  of  the 
changes  of  turgidity  which  cause  the  circumnutation. 
Each  band  is  in  a  particular  phase  of  its  rhythm  at  any 
given  moment,  and  the  successive  bands  follow  one  another 
through  the  phases  of  their  rhythm  in  orderly  sequence,  so 
that  when  one  is  at  its  maximum,  another  diametrically 
opposite  to  it  is  at  its  minimum.  The  phases  of  maximum 
and  minimum  turgidity  thus  pass  rhythmically  round  the 
organ,  and  the  apex  is  consequently  compelled  to  describe 
a  spiral  line  as  it  grows.  If  the  stem  or  root  is  not  circular 
in  section,  but  is  flattened  in  any  direction,  the  steady 
sequence  of  the  rhythmic  changes  will  cause  the  projection 
of  this  spiral  to  assume  the  form  of  an  ellipse  instead  of  a 
circle,  and  if  the  flattening  is  extreme  the  movement  will 
be  a  backward  and  forward  one. 

Modifications    of    the    distribution    of    maximum    and 


EHYTHM  361 

minimum  turgescence  in  a  radially  symmetrical  organ  may 
lead  to  a  similar  nutation.  It  is  not  infrequent  for  the 
rhythmic  change  in  the  turgescence  to  affect  only  two  sides, 
instead  of  passing  regularly  round  it.  The  organ,  though 
radially  symmetrical  in  structure,  will  thus  behave  as  a 
bilaterally  symmetrical  one,  its  organisation  indeed  being 
bilaterally  symmetrical.  Its  changes  of  position  will  thus 
resemble  those  of  a  flattened  organ  which  can  only  be  made 
to  oscillate  backwards  and  forwards. 

A  similar  rhythm  can  be  noticed  in  the  variations  of  the 
extensibility  of  the  limiting  membrane  which  characterise 
the  circumnutation  of  a  coenocytic  hypha.  We  must  sup- 
pose these  variations  to  be  due  to  the  protoplasm  covering 
the  wall,  though  we  cannot  explain  the  mechanism.  The 
protoplasm  has  the  power  to  soften  the  cell-membrane. 

Khythmic  changes  of  this  kind  affect  other  processes 
than  those  of  circumnutation.  We  have  had  occasion  to 
notice  that  the  behaviour  of  a  growing  organ  during  its 
grand  period  shows  certain  diurnal  variations  which  we 
have  called  the  daily  periodicity  of  growth.  Though  no 
doubt  we  have  to  do  here  to  a  certain  extent  with  changes 
in  the  behaviour  of  the  protoplasm  induced  by  the  alter- 
nations of  light  and  darkness,  with  coincident  variations 
in  temperature,  this  daily  periodicity  of  the  rhythm  does 
not  appear  to  be  altogether  dependent  upon  exposure  to 
such  alternations,  for  they  persist  for  a  longer  or  shorter 
time  during  continuous  darkness.  Their  cessation  after 
exposure  to  a  period  of  darkness  need  not  necessarily 
point  to  their  dependence  on  the  intermittent  access  of 
light  and  warmth,  for,  as  we  shall  see  later,  prolonged 
deprivation  of  light  leads  to  a  peculiar  condition  of  rigidity 
of  the  protoplasm  which  eventually  causes  its  death.  The 
cessation  of  the  rhythm  indeed  appears  to  be  a  pathological 
phenomenon.  The  rhythm  of  the  daily  periodicity  appears, 
however,  to  bear  a  certain  relationship  to  the  alternation 
of  day  and  night,  for  plants  which  have  been  cultivated 
from  seed  in  continuous  darkness  do  not  exhibit  it. 


362  VEGETABLE  PHYSIOLOGY 

This  rhythmic  change  in  the  protoplasm  is  not  exhibited 
by  organs  during  growth  only  ;  in  many  cases  it  persists 
throughout  their  life.  Very  conspicuous  instances  of  it  are 
afforded  by  certain  movements  often  exhibited  by  the  leaves 
of  particular  plants.  Perhaps  the  most  familiar  of  these  is 
the  so-called  Telegraph  plant,  Desmodium  or  Hedysarum 
gyrans.  Its  leaves  are  ternate,  the  terminal  leaflet  a  being 
very  large  in  comparison  with  the  two  lateral  ones  b  (fig.  151). 
If  the  plant  is  watched  while  exposed  to  suitable  tempera- 
ture and  illumination,  the  lateral  leaflets  are  found  to  move 
up  and  down  on  the  rachis,  sometimes  passing  through  an 

angle  of  180°,  and  twisting  slightly 
as  they  move.  They  thus  describe 
a  kind  of  ellipse,  the  duration  of 
the  movement  being  about  two 
minutes.  Many  other  instances  of 
a  similar  kind  are  known,  the 
Leguminoscp  furnishing  many  ex- 
amples. All  of  them  do  not  exhibit 
the  movements  with  the  same  ease, 
as  they  are  interfered  with  by 
other  changes  in  position  which 
FIG.  161.— TERNATE  LEAF  OF  result  from  external  stimulation. 

THE  TELEGRAPH  PLANT  (Des-       mi  ,,        ,  ,          .-1,1 

medium  gyrans).  They  can  often  be  made  evident  by 

keeping  the  plant  under  constant 

external  conditions.  Darkness,  however,  if  too  prolonged 
causes  their  cessation,  though  in  some  cases  they  are  made 
evident  by  deprivation  of  light  for  a  short  time.  The 
mechanism  of  the  movement  in  most  of  these  cases  is 
the  rhythmically  varying  turgescence  of  particular  organs 
known  as  pulvini,  which  are  situated  at  the  bases  of  the 
stalks  of  the  leaves  or  leaflets.  As  in  the  cases  already 
noticed,  the  alternations  in  the  turgescence  are  the  expres- 
sion of  rhythmic  changes  in  the  protoplasm  of  the  cells  of 
each  pulvinus.  As  these  pulvini  play  a  considerable  part  in 
the  changes  of  position  which  are  exhibited  by  many  leaves 
under  various  conditions,  their  structure  may  well  attract 


KHYTHM 


363 


our  attention  here.  Fig.  152  represents  a  longitudinal 
section  through  one  of  them,  which  occurs  at  the  base  of 
a  leaf  of  Mimosa.  The  stalk  of  the  leaf  shows  a  swelling 
at  the  point  of  union  with  the  stem,  the  protuberance 
being  greatest  on  the  under  side.  Here  there  is  a  cushion 


FIG.  152.— PULVINUS  OF  Mimosa. 

a,  b,  the  succulent  parenchyma  of  its  upper  and  lower  sides ;    c,  bud ; 
d,  parenchyma  of  stem  ;   e,  pith. 

of  cells  which  are  capable  of  containing  a  relatively  con- 
siderable quantity  of  water.  When  turgid  they  swell  out 
and  force  the  leaf  into  an  erect,  or  almost  erect,  position. 
When  they  part  with  their  water  and  become  flaccid,  the 
stalk  of  the  leaf  loses  its  support  and  the  weight  of  the  blade 
causes  it  to  fall  downwards.  This  is  rendered  more  easy 
by  the  fact  that  the  vascular  strand  or  bundle  which  passes 
from  the  stele  of  the  stem  through  the  petiole  is  somewhat 


364  VEGETABLE  PHYSIOLOGY 

reduced,  giving  greater  flexibility  to  the  stalk  at  that  point. 
The  cells  upon  the  upper  side  of  the  pulvinus  in  some  cases 
play  only  a  passive  part  in  the  phenomenon,  the  rhythmic 
variations  only  affecting  those  already  described  ;  in  other 
cases  both  sides  may  show  the  changes  of  turgidity. 

The  same  tendency  to  rhythmic  change  is  shown  in  what 
is  called  the  periodicity  of  the  various  vital  functions.  If, 
for  instance,  the  root-pressure  of  a  plant  is  examined  by  the 
"aid  of  the  apparatus  already  described,  in  which  the  water 
taken  up  is  made  to  support  a  column  of  mercury  in  a 
manometer,  when  the  mercury  has  reached  what  we  may 
call  its  mean  or  average  height,  it  does  not  remain  steady  at 
that  point,  but  begins  to  oscillate.  It  rises  in  the  morning 
till  about  midday,  then  sinks  somewhat,  rises  again  during 
the  evening,  and  falls  during  the  night.  There  is  thus 
a  daily  variation  of  the  absorptive  activity  of  the  roots 
which  is  scarcely  affected  by  changes  in  the  environment. 
It  is  an  instance  of  an  automatic  rhythm. 

There  is  a  similar  daily  variation  in  the  bulk  of  a  plant, 
the  diameter  of  its  various  organs  diminishing  from  night 
till  some  time  in  the  afternoon,  and  increasing  thenceforward 
till  dawn.  These  variations  largely  depend  upon  the  dis- 
tribution of  the  water  which  the  plant  contains,  which  is 
regulated  by  the  living  substance  in  the  way  already 
described.  This  rhythm  is  under  ordinary  circumstances 
very  much  affected  by  variations  in  transpiration,  which 
we  have  seen  is  a  process  that  is  very  soon  modified  by 
variations  in  illumination  and  temperature. 

It  is  difficult  to  explain  the  occurrence  of  these  various 
manifestations  of  rhythmic  change  in  the  protoplasm.  Many 
of  them  suggest  that  they  are  the  result  of  the  influence  of 
the  alternations  of  light  and  darkness,  and  perhaps  of  the 
changes  of  the  seasons,  to  which  plants  are  exposed.  But 
others  are  exhibited  so  regularly  under  constant  conditions 
of  the  environment  that  they  cannot  be  thus  explained. 
They  are  now  hereditary  and  to  a  certain  extent  indepen- 
dent of  the  changing  conditions  which  the  plants  encounter. 


SENSITIVENESS  365 

It  may  well  be,  however,  that  they  have  become  impressed 
upon  the  organisation  of  vegetable  protoplasm  by  the  con- 
stant recurrence  of  these  changes  of  the  environment  during 
the  long  ages  of  the  past.  This  does  not  appear  unlikely 
in  the  face  of  the  fact  that,  as  we  shall  see  later,  it  is  possible 
under  appropriate  conditions  to  impress  a  new  rhythm 
upon  particular  organs.  The  manifestation  of  rhythmic 
change  has,  however,  become  one  of  the  vital  properties  of 
protoplasm. 

We  saw  in  an  earlier  chapter  that  the  peculiarities  of 
form  and  structure  which  different  plants  possess  are  to  be 
associated  with  the  character  of  their  environment.  From 
such  facts  as  were  there  discussed  it  is  evident  that  a  plant 
is  capable  of  receiving  impressions  from  without  and 
responding  to  them  in  various  ways.  If  we  examine  any 
plant  which  does  not  show  such  marked  adaptation  to  its 
surroundings  as  those  which  were  then  more  particularly 
under  consideration,  we  can  still  find  evidence  of  the  pos- 
session of  a  similar  power  of  appreciating  differences  in  the 
external  conditions  in  which  it  finds  itself,  and  of  modifying 
certain  of  its  vital  processes  in  response.  When  certain 
zoospores  of  some  of  the  lower  Algae  which  swim  freely  in 
water  are  suddenly  exposed  to  a  brilliant  light,  they  take 
up  at  once  a  definite  position  with  regard  to  the  incident 
rays.  When  a  leaf  of  Mimosa  pudica,  the  so-called  sensi- 
tive plant,  is  roughly  handled,  it  falls  from  its  normal  position 
and  takes  up  a  new  one,  while  its  leaflets  become  folded 
together.  When  a  filament  of  Mesocarpus  is  exposed  to 
an  electric  shock  sent  through  the  water  in  which  it  is 
floating,  it  is  found  not  infrequently  that  it  splits  up  into 
its  constituent  cells.  This  power  of  receiving  impressions 
from  without,  to  which  we  have  had  frequently  to  refer  in 
discussing  the  phenomena  of  growth  and  rhythm,  is  another 
property  of  vegetable  protoplasm,  and  can  be  observed 
to  belong,  in  a  greater  or  less  degree,  to  every  vegetable 
organism.  It  is  usually  spoken  of  under  the  general  term 
irritability  or  sensitiveness. 


866  VEGETABLE  PHYSIOLOGY 

This  property  is  not  always  to  be  observed  or  demon- 
strated with  equal  ease.  Indeed,  the  protoplasm  must  be  in 
a  healthy  condition  to  manifest  it  satisfactorily.  It  is  easily 
injured  if  changes  in  the  environment  are  too  sudden  or  too 
severe.  Consequently  the  adaptation  of  groups  of  plants  to 
special  environments  has  been  a  slow  and  difficult  process, 
any  single  individual  undergoing  little  change,  but  altera- 
tions of  considerable  extent  have  been  effected  by  the  con- 
tinuous influencing  of  many  generations. 

The  maintenance  of  the  health  of  the  individual  is  no 
doubt  the  great  object  of  this  sensitiveness  ;  and  conversely 
it  is  only  the  healthy  plant  that  manifests  it  in  the  greatest 
fulness.  Health  may  be  spoken  of  as  the  condition  in 
which  the  reaction  between  an  organism  and  its  surround- 
ings is  a  perfect  one.  In  the  case  of  the  ordinary  terrestrial 
plant  these  surroundings  present  especially  three  features 
which  are  subject  to  considerable  variation.  These  are 
light,  temperature,  and  moisture.  A  plant  must  exhibit 
a  proper  relationship  to  each  of  these  conditions,  at  any 
rate,  to  be  healthy.  The  condition  in  which  the  relationship 
to  each  of  these  factors  is  satisfactory  is  generally  spoken 
of  as  one  of  tone,  and  the  influence  which  each  exerts  when  it 
affects  the  plant  uniformly  is  spoken  of  as  a  tonic  influence. 
When  a  dicotyledonous  plant  which  has  been  growing  under 
ordinary  atmospheric  conditions,  exposed  to  diffused  day- 
light, is  removed  into  darkness  and  kept  there  for  some 
time,  it  becomes  incapable  of  being  impressed  by  its  sur- 
roundings. Nor  is  its  irritability  alone  affected  by  the 
absence  of  light,  for  many  of  its  parts,  particularly  its  leaves, 
cease  to  grow  under  such  conditions.  The  condition  which 
is  induced  by  light,  and  upon  which  the  manifestations  of 
irritability  depend,  is  known  as  Phototonus.  It  indicates 
a  certain  effort  on  the  part  of  the  protoplasm  to  adjust 
itself  to  the  intensity  of  the  illumination. 

A  corresponding  condition,  marking  an  appropriate 
relationship  between  the  plant  and  temperature,  may  be 
called  Thermotonus.  This  condition" also  is  necessary  for 


TONE  367 

the  manifestation  of  sensitiveness.  If  it  is  materially 
interfered  with,  the  vital  functions  and  the  processes  of 
growth  and  nutrition  suffer  seriously. 

There  must  also  be  a  satisfactory  adjustment  of  the 
relations  between  a  plant  and  moisture,  though  this  is  less 
restricted  than  the  two  already  mentioned. 

As  the  maintenance  of  health  involves  the  continual 
adjustment  of  the  plant  to  the  changes  in  its  environment, 
we  must  examine  a  little  more  closely  the  nature  of  the 
influence  which  the  latter,  and  particularly  the  two  factors 
of  light  and  temperature,  exert  upon  the  organism.  This 
influence  is  spoken  of  as  a  tonic  or  paratonic  influence,  and 
leads  to  the  establishment  of  a  satisfactory  condition  of 
tone. 

In  order  to  study  the  tonic  influence  of  light  upon  a 
plant  we  may  first  consider  the  features  which  characterise 
the  growth  of  a  plant  in  darkness.  We  find  that  such  a 
plant  is  much  modified  both  in  form  and  structure.  If  we 
experiment  with  an  ordinary  dicotyledonous  plant  which 
has  numerous  leaves  of  moderate  or  small  size  upon  an 
elongated  stem,  we  find  that  these  features  become  much 
exaggerated.  The  stem  becomes  very  much  elongated  and 
remains  slender ;  it  is  more  succulent  than  a  normal  stem, 
and  bears  extremely  small  leaves  which  grow  out  from  it 
at  a  more  acute  angle  than  those  which  rise  upon  a  normally 
illuminated  stem.  Certain  Monocotyledons  which  have 
normally  small  stems  and  large  broad  leaves  are  differently 
affected.  The  great  change  in  this  case  is  in  the  leaves, 
which  become  much  elongated  and  relatively  narrower 
than  normal  ones.  Certain  phylloclades,  such  as  those  of 
some  of  the  Cacti,  become  elongated  and  slender,  instead 
of  remaining  broad. 

The  structure  of  the  various  parts  also  is  modified  ;  the 
woody  and  sclerenchymatous  elements  are  much  reduced, 
and  the  parenchyma  of  the  cortex  is  increased  in  bulk.  It 
becomes  more  succulent,  and  the  reaction  of  its  sap  is  much 
more  acid.  The  chloroplasts  do  not  become  green,  the 


368  VEGETABLE  PHYSIOLOGY 

pigment  which  they  contain,  known  as  etiolin,  being  a  pale 
yellow.  In  the  leaves  the  differentiation  of  the  mesophyll 
into  palisade  and  spongy  parenchyma  does  not  take  place. 
The  parenchymatous  cells  of  the  ground  tissue  of  the 
elongated  organs,  whether  they  are  stems  or  leaves,  become 
altered  in  shape,  their  longitudinal  diameter  being  con- 
siderably increased.  Plants  thus  affected  by  darkness  are 
said  to  be  etiolated. 

That  these  differences  are  to  be  attributed  to  the  absence 
of  the  light  can  be  seen  by  comparing  two  similar  plants, 
the  first  cultivated  in  darkness  and  the  second  under 
ordinary  conditions  of  illumination,  the  other  conditions 
being  kept  the  same  for  both. 

The  explanation  of  these  changes  is  somewhat  difficult. 
The  absence  of  light  is  clearly  the  cause  of  the  different 
colour,  for,  as  we  have  seen  in  a  preceding  chapter,  under 
such  conditions  the  pigment  chlorophyll  is  not  formed,  but 
is    replaced    by    the    yellowish- white    etiolin.    When    an 
etiolated  plant  is  exposed  to  light,  the  etiolin  is  soon  re- 
placed by  chlorophyll,  and  the  plant  becomes  green.     The 
question  of  the  non- development  of  the  woody  elements 
and  the  generally  increased  succulence  is  more  difficult  to 
explain,   and   many  hypotheses   have   been   advanced   to 
account  for  it.     There  is  a  disturbance  of  the  normal  course 
of   the   metabolism   evidently,   as   shown   by   the   greater 
production  or  accumulation  of  organic  acids,  to  the  osmotic 
properties  of  which  the  increased  succulence  is  partly  due. 
It  is  known  that  in  plants  possessing  considerable  succu- 
lence the  free  organic  acids  which  are  produced  during  the 
night  undergo  oxidation  when  light  finds  access  to  them. 
The  reason  for  the  disturbance  in  question  is,  however, 
not    explained.      Diminished    transpiration    may    perhaps 
account  for  a  good  deal,  for,  as  we  have  seen,  in  the  absence 
of  light  the  stomata  remain  shut  and  there  is  but  little 
output  of  watery  vapour.     The  increased  turgidity  of  the 
tissues  resulting  from  this  factor  may  very  probably  upset 
the  normal  course  of  metabolism. 


ETIOLATION  369 

It  is  significant  in  this  connection  that  the  parts  which 
show  the  excessive  growth  are  in  all  cases  those  in  which 
water  accumulates  as  transpiration  becomes  checked. 

If,  however,  the  effects  are  admitted  to  be  due  to  the 
disturbance  of  transpiration,  this  is  no  satisfactory  or  final 
explanation  of  the  phenomenon,  for,  as  we  have  seen,  the 
actual  evaporation  of  the  water  of  transpiration  into  the 
intercellular  spaces  is  under  the  regulating  influence  of 
the  protoplasm,  and  the  effect  must  therefore  be  traced  back 
to  some  interference  with  the  latter,  caused  by  the  absence  of 
illumination.  With  the  lowering  of  the  tone  which  follows 
the  absence  of  the  light  we  have  a  failure  of  the  protoplasm 
to  exhibit  its  normal  degree  of  permeability,  which  is 
maintained  by  some  slight  effort,  and  we  find  it  retains  in 
the  cell  more  than  the  usual  quantity  of  water. 

We  cannot  easily  explain  the  effects  which  we  have 
seen  are  produced  upon  the  structural  elements  of  the 
plant.  We  do  not  know  why  the  usual  development  of  the 
woody  and  sclerenchymatous  cells  of  the  stem  should  be 
interfered  with,  nor  can  we  explain  the  effect  of  light  upon 
the  degree  of  differentiation  of  the  mesophyll  of  leaves. 
We  find  that  palisade  tissue  is  developed  more  readily 
under  the  influence  of  bright  light,  a  phenomenon  which 
may  be  easily  ascertained  by  comparing  the  structure  of 
two  leaves  of  the  same  tree,  one  taken  from  a  brilliantly 
lighted  and  the  other  from  a  deeply  shaded  part.  Indeed, 
the  differentiation  of  the  mesophyll  into  palisade  and 
spongy  parenchyma  may  be  traced  to  the  difference  of 
illumination  which  the  two  faces  of  a  leaf  receive,  for  when 
both  are  well  lighted,  palisade  parenchyma  appears  upon 
both  sides ;  while  etiolated  leaves,  as  we  have  seen,  do  not 
develop  this  tissue  at  all. 

It  is  possible  that  this  difference  of  structure  on  the 
two  sides  may  be  connected  with  the  possibility  of  damage 
to  the  chloroplasts  if  they  are  too  brilliantly  illuminated. 
The  arrangement  of  the  palisade  cells  shields  them  to  a 
certain  extent. 

24 


370  VEGETABLE  PHYSIOLOGY 

If  we  pass  to  consider  the  effects  of  too  intense  an  illumina- 
tion we  find  that  it  is  attended  with  considerable  danger 
to  the  well-being  of  the  protoplasm.  When  the  leaves 
of  certain  plants,  among  which  may  be  mentioned  Oxalis 
acetosella,  are  kept  exposed  to  very  strong  sunlight,  and 
prevented  from  shading  themselves  as  they  normally  do 
by  changes  in  their  position,  they  rapidly  die,  the  dura- 
tion of  their  life  being  reduced  from  two  or  three  months 
"to  as  many  days.  Bright  sunlight  has  in  other  cases  been 
found  to  check  the  growth  in  length  of  seedlings,  the  effect  of 
different  degrees  of  illumination  having  been  compared  by 
direct  measurement.  We  find  various  arrangements  in 
different  plants  which  appear  to  be  directed  towards  pro- 
tecting them  from  the  effects  of  too  brilliant  an  insolation. 
Many  which  normally  have  their  leaves  so  arranged  as  to 
expose  their  upper  surfaces  to  the  incident  rays  are  found 
under  bright  surlight  to  place  them  so  that  their  edges 
and  not  their  surfaces  receive  the  light.  This  phenomenon 
has  been  called  Pardheliotropi&m.  It  is  exhibited  normally 
by  the  leaves  of  Oxalis  which  have  just  been  alluded  to. 

Another  phenomenon,  having  for  its  purpose  the  protec- 
tion of  the  chlorophyll,  can  be  seen  in  many  ordinary  dorsi- 
ventral  leaves.  When  brightly  illuminated  they  are  of  a 
lighter  green  colour  than  when  shaded,  and  this  has  been 
found  to  be  due  to  a  different  arrangement  of  the  chloro- 
plasts  in  the  two  cases.  In  a  leaf  exposed  to  diffused  light 
these  are  collected  on  the  upper  and  lower  walls  of  the  cells 
just  under  the  epidermis,  and  they  present  their  broader 
surfaces  to  the  incident  rays.  When  the  light  is  cut  off 
altogether  for  a  considerable  time,  and  other  conditions 
are  unfavourable,  they  collect  on  the  lateral  and  lower 
walls.  When  the  leaf  is  brilliantly  illuminated  they  place 
themselves  upon  the  lateral  walls  only,  and  rotate  on  their 
long  axis  so  as  to  present  their  edges  instead  of  their  sur- 
faces to  the  light.  In  the  first  case  the  chloroplasts  lie 
parallel  to  the  surface  of  the  leaf,  and  receive  as  much 
light  as  they  can ;  in  the  last  they  lie  at  right  angles  to 


INFLUENCE  OF  LIGHT  ON  PROTOPLASM    371 

the  surface  so  as  to  receive  as  little  as  possible.  These 
two  conditions  are  known  as  epistrophe  and  apostrophe 
respectively.  When  the  conditions  of  the  incidence  of  the 
light  are  altered,  the  chloroplasts  change  their  positions 
accordingly. 

The  Alga  Mesocarpus  exhibits  the  phenomenon  in  a  very 
striking  manner.  It  consists  of  somewhat  ohlong  or  slightly 
elongated  cells  arranged  in  a  filament.  Each  cell  contains 
a  single  band-like  chloroplast  which  lies  nearly  parallel 
to  the  long  axis  of  the  cell.  In  ordinary  daylight  it  places 
itself  so  that  the  surface  of  the  band  is  exposed  to  the  illu- 
minating rays,  but  if  the  light  becomes  intense,  it  revolves 
quickly  upon  its  long  axis,  so  that  its  edge  is  presented  to 
them. 

A  different  effect  of  a  strong  light  is  manifested  by  many 
dor  si  ventral  structures,  of  which  the  thallus  of  Marchantia 
affords  a  good  example.  Whichever  side  of  the  organ 
is  brilliantly  illuminated,  the  dorsal  or  upper  surface  shows 
accelerated  growth,  so  that  the  thallus  exhibits  epinasty. 
Some  of  the  radially  symmetrical  structures  which  have 
been  mentioned  as  bilaterally  organised  (page  361)  behave 
similarly.  Such  are  runners  of  Polygonum  aviculare,  and 
other  plants  of  similar  habit.  This  phenomenon  has  been 
called  photo-epinasty,  as  the  increased  growth  of  the  dorsal 
side  is  due  to  the  access  of  light. 

These  facts  may  perhaps  give  us  some  idea  of  the  influence 
of  light  upon  protoplasm,  and  the  condition  of  tone,  one  of 
whose  chief  features  is  the  proper  regulation  of  the  per- 
meability of  the  protoplasm  by  water.  In  darkness  meta- 
bolism and  growth  are  greatly  affected,  the  latter  being 
unduly  accelerated.  In  the  presence  of  too  strong  a  light, 
a  deleterious  influence  is  exerted.  An  intermediate  con- 
dition exists  in  which  the  vital  processes  of  growth  and 
nutrition  and  the  sensitiveness  to  external  influences  are 
seen  at  their  best.  This  is  the  condition  of  tone  or  photo- 
tonus,  and  its  maintenance  may  be  spoken  of  as  due  to  the 
tonic  influence  of  light.  It  is  frequently  said  that  light 

24* 


372  VEGETABLE  PHYSIOLOGY 

retards  growth,  and  the  tonic  influence  is  associated  with 
this  retardation.  This  is,  however,  a  somewhat  incomplete 
presentation  of  the  case.  Eetardation  of  the  growth  is  not 
the  only  effect  produced  by  the  access  of  a  proper  degree 
of  illumination.  It  is  rather  to  be  regarded  as  regulatory 
than  retarding,  and  as  it  affects  many  other  functions 
than  growth,  it  seems  more  appropriate  to  consider  the 
influence  of  the  light  as  directed  to  the  maintenance  of  this 
"  tone,  which  is  really  one  of  the  conditions  of  health.  How 
the  actual  effect  upon  the  protoplasm  is  produced  we  cannot 
say ;  it  may  be  that  the  motility  which  is  characteristic  of 
healthy  protoplasm  and  its  control  of  its  own  permeability 
are  adjusted  to  a  particular  relationship  with  the  environ- 
ment, of  which  phototonus  is  one  condition. 

The  rays  of  the  spectrum  which  exert  this  influence  on 
the  living  substance  appear  to  be  those  of  high  refrangibility, 
the  blue  and  the  violet.  To  these  rays  the  protoplasm  seems 
to  be  excessively  sensitive.  We  do  not  explain  their  action 
when  we  say  that  they  bring  about  a  variation  in  the  turgidity 
of  the  cells,  or  that  they  set  up  a  change  in  the  manner  of  the 
nutrition.  The  facts  which  we  have  called  attention  to  can 
only  be  referred  to  the  power  of  the  protoplasm  to  respond 
to  their  influence. 

The  question  of  the  influence  of  temperature  upon  the 
tone  of  the  plant  need  not  here  be  considered  so  fully,  as 
in  a  preceding  chapter  we  have  discussed  the  phenomena 
of  the  general  relations  of  temperature  to  the  plant  at 
some  length.  We  may,  however,  again  point  out  that 
plants  are  affected  by  variations  in  temperature  in  ways 
very  similar  to  those  depending  on  changes  in  light.  It  is 
not,  however,  always  easy  to  ascertain  the  effects  due  to 
changes  in  temperature  alone,  as  other  conditions,  such  as 
light  and  moisture,  usually  vary  at  the  same  time  as  the 
temperature  changes. 

As  we  have  seen,  the  environment  of  the  plant  is  partly 
the  soil  and  partly  the  atmosphere,  and  the  temperature  of 
both  may  or  may  not  vary  simultaneously.  We  have 


TONE  373 

seen  that  for  each  metabolic  process  there  is  a  temperature 
at  which  it  progresses  to  the  greatest  advantage.  At  lower 
and  at  higher  points  the  protoplasm  is  less  active,  and  in 
each  case  there  is  a  point  below  which  activity  ceases,  and 
one  above  which  also  it  does  not  go  on.  The  same  thing, 
we  have  seen,  is  true  of  the  processes  of  growth.  We  may 
say  that  for  each  plant  there  is  a  particular  temperature  at 
which  it  carries  out  the  aggregate  of  its  functions  most 
advantageously,  and  it  is  when  exposed  to  this  temperature 
that  it  is  in  a  condition  of  the  most  complete  thermotonus. 
This  point  is  not  the  same  for  every  plant,  indeed  consider- 
able differences  exist  in  this  respect.  We  may  say,  more- 
over, that  it  is  perhaps  not  so  much  a  point  as  a  range  of 
temperature,  for  small  divergences  from  the  actual  optimum 
point  have  but  little  effect  upon  the  tone.  Within  this 
range  the  constant  round  of  activity,  chemical  and  physical, 
which  is  the  expression  of  life,  goes  on  most  advantageously  ; 
below  it  it  is  injuriously  affected,  and  at  a  minimum  point 
it  is  suspended.  At  another  point,  higher  in  the  scale, 
spoken  of  as  the  maximum  temperature,  the  death  of  the 
protoplasm  usually  ensues. 

We  cannot  explain  the  influence  of  temperature  upon 
the  protoplasm  any  more  satisfactorily  than  we  can  that  of 
light.  All  we  know  is  that  the  two  co-operate  together  to 
keep  the  plant  in  the  condition  to  which  we  have  given  the 
name  of  health. 

The  tone  of  the  plant  depends  very  greatly  upon  a  proper 
adjustment  of  the  relations  between  the  protoplasts  and 
water.  For  the  maintenance  of  health  it  is  essential  that 
the  normal  turgidity  of  the  cells  shall  not  be  disturbed. 
A  definite  amount  of  hydrostatic  pressure  inside  such  cells  is 
necessary,  as  we  have  seen,  for  the  due  or  efficient  discharge 
of  the  processes  of  life.  We  may  regard  the  maintenance 
of  this  relationship  as  one  of  the  chief  features  of  tone, 
for  it  involves  a  particular  condition  of  the  protoplasm 
with  regard  to  its  permeability.  This  condition  may  be 
regarded  as  a  kind  of  effort,  the  living  substance  exerting 


374  VEGETABLE  PHYSIOLOGY 

some  active  living  influence  comparable  to  the  condition 
of  almost  passive  contraction,  which  is  the  normal  condition 
of  various  muscular  structures  in  the  animal  body.  The 
effort  seems  to  be  directed  to  reducing  the  resistance  its 
structure  offers  to  the  passage  of  water  through  it.  If  it 
is  increased,  the  existing  hydrostatic  pressure  causes  an 
excessive  escape  of  water,  and  the  cells  become  flaccid  ;  if 
it  is  relaxed,  the  normal  interchange  of  water  between  cells 
is  diminished  to  their  detriment,  the  permeability  of  the 
protoplasm  becoming  lessened. 

A  further  aspect  of  tone  may  be  seen  to  depend  upon  a 
constant  and  regular  supply  of  oxygen.  The  function  of 
this  gas  in  vegetable  life  has  already  been  discussed  at 
some  length  in  a  preceding  chapter.  We  have  seen  that 
if  its  access  is  interfered  with  the  whole  organism  is  for  a 
time,  if  not  permanently,  upset,  all  the  vital  functions 
being  thrown  into  disorder.  The  power  of  appreciating 
and  responding  to  stimulation  is  also  lost. 

Another  property  which  vegetable  protoplasm  possesses, 
and  which  is  of  the  highest  importance  in  adapting  the 
organism  to  its  environment,  is  what  has  been  termed 
acclimatisation.  This  is  manifested  by  the  fact  that  after 
long  continued  applications  of  a  particular  stimulus  the 
organism  ceases  to  respond  to  it.  We  can  frequently 
notice  that  a  plant,  accustomed  to  live  in  light  of  but  feeble 
intensity,  if  made  to  grow  in  a  brighter  region,  though 
injuriously  affected  at  first,  will  ultimately  thrive  in  it  as 
well  as  it  did  before.  Similar  phenomena  in  connection 
with  temperature  have  been  observed. 


375 


CHAPTEE  XXIII 

STIMULATION    AND    ITS   RESULTS 

We  may  gather  from  what  has  just  been  said  that  there 
may  exist  for  every  plant,  at  any  rate  theoretically,  a  con- 
dition of  adjustment  when  it  is  in  absolute  harmony  with 
its  environment,  and  when,  consequently,  its  life  is  being 
regulated  to  the  utmost  advantage.  We  can  see,  however, 
that  such  a  condition  can  be  only  momentary  in  any  case, 
for  the  environment  is  in  a  constant  state  of  change  and  the 
protoplasm  of  the  organism  is  also  exhibiting  continual 
mobility.  For  the  maintenance  of  health,  or  even  of  life, 
it  is  essential  that  variations  in  one  shall  be  adequately 
responded  to  by  variations  in  the  other,  and  the  impossi- 
bility of  securing  indefinitely  such  a  continual  adjustment 
of  relations  is  the  cause  of  the  cessation  of  life. 

The  responses  which  the  organism  makes  to  such  altera- 
tions in  its  surroundings  may  now  be  considered  in  greater 
detail,  and  we  may  thereby  form  some  acquaintance  with 
the  causes  which  have  led  to  such  great  diversities  in  form, 
structure,  and  habit  of  life  as  we  have  already  seen  to 
characterise  large  groups  of  plants. 

Any  change  in  the  environment  which  provokes  some 
alteration  of  behaviour  on  the  part  of  a  plant  is  spoken  of 
as  a  stimulus,  and  the  change  of  behaviour  is  to  be  looked 
upon  as  the  result  of  stimulation.  When  we  come,  however, 
to  define  more  narrowly  what  we  understand  by  the  terms 
stimulus  and  stimulation,  we  find  it  is  not  easy  to  restrict 
them  to  such  changes  in  the  surroundings  as  we  are  able 


376  VEGETABLE  PHYSIOLOGY 

to  observe  and  perhaps  measure  by  even  the  most  delicate 
instruments  at  our  disposal. 

Many  changes  take  place  in  protoplasm  which  escape 
our  observation,  originating  perhaps  in  the  condition  of  the 
protoplasm  itself,  or  being  due  to  disturbances  in  the  interior 
of  the  plant.     The  normal  course  of  metabolism  may  under- 
go a  marked  change  in  consequence  of  variation  in  the 
amount  of  some  particular  constituent  of  the  food,  or  of  an 
alteration  of  the  distribution  or  direction  of  the  transloca- 
tory  stream.     Injury  to  the  body  of  the  plant  may  involve 
redistribution  of  energy  or  of  material  within  its  interior, 
which  may  have  far-reaching  effects  upon  the  course   of 
the  vital  processes.     Variations  in  the  supply  of  food,  which 
may  range  between  absolute  starvation  and  over-engorge- 
ment, may  produce  very  great  changes  not  only  in  the  outer 
life  of  the  plant,  but  in  the  substances  it  produces  in  its 
metabolism  and  the  energy  which  it  liberates.     The  lack  of 
oxygen  may  provoke  an  almost  entirely  new  metabolism 
in  connection  with  the  production  of  such  energy.     These 
internal    changes    have   been   already   discussed,   and  the 
effect  of  various  factors  at  work  in  the  organism  have  been 
examined,  so  that  it  is  not  necessary  in  the  present  connec- 
tion to  do  more  than  emphasize  the  fact  that  we  have 
in  such  matters  evidence  of  stimulation  and  the  response 
it  provokes — evidence  which  points  to  the  sensitiveness  or 
irritability  of  protoplasm,  as  much  as  do  the  results  of 
those  changes  in  the  environment  which  are  purely  external. 
The  internal  stimuli  just  noticed  are  largely  chemical  in 
character,  and  though  chemical  changes  in  the  protoplasm 
are  continuously  occurring,  many  of  them  are  directly  in- 
stigated by  such  stimuli.     Whether  the  automatic  changes  in 
organs  and  cells  which  we  have  already  studied  are  due  to 
stimulation  is  perhaps  a  little  doubtful ;  but  at  any  rate  the 
nature  of  any  stimulus  provoking  them  has  so  far  eluded 
investigation,  and  to  all  appearances  they  are  not  initiated 
in  that  way,  but  are  independent  of  all  stimulation. 

Stimulation  which  is  directly  due  to  the  physical  condi- 


STIMULATION  AND  ITS  EESULTS  377 

tions  of  the  environment  may  be  looked  upon  as  the  effect 
of  any  modification  of  the  conditions  which  have  induced 
tone.  We  have  seen,  for  instance,  that  a  particular  degree 
or  range  of  illumination  sets  up  in  a  plant  the  condition 
of  phototonus,  which  is  one  constituent  of  the  healthy  tone 
of  the  organism.  Any  modification  of  that  illumination  is 
followed  by  certain  effects,  the  extremes  of  which  we  have 
already  discussed.  This  alteration  of  the  optimum  illu- 
mination becomes  at  once  a  stimulating  action,  and  we  can 
speak  of  a  stimulating  influence  of  light,  which  is  really 
any  change  in  what  we  have  called  its  tonic  action.  It  can 
be  in  the  direction  of  increase  or  decrease  of  the  latter,  but 
as  it  induces  changes  it  must  be  regarded  as  stimulating. 

What  is  true  of  light  is  also  true  of  the  other  factors 
which  combine  to  produce  the  healthy  tone  of  the  plant. 
Changes  of  temperature  bring  the  organism  nearer  to  or 
farther  from  that  optimum  point  at  which  it  is  in  the  most 
complete  state  of  thermotonus,  and  are  responded  to  in 
various  ways  accordingly.  Any  alteration  in  the  fluid 
contents  of  a  cell  brings  about  a  change  in  what  we  may 
call  the  tonic  tension  of  that  cell,  in  which  condition  the 
permeability  of  the  protoplasm  exists  at  its  best,  and  again 
an  appropriate  response  is  made. 

In  considering  broadly  the  result  of  stimulation  we 
must  notice  at  the  outset  that  it  provokes  a  purposeful 
response.  The  living  substance  appears  to  have  a  definite 
aim  ;  it  may  be  to  remove  the  stimulating  cause  if  the 
latter  affects  it  prejudicially ;  it  may  be  to  readjust  its 
manifold  forces  to  the  new  conditions  to  which  the  environ- 
ment is  suddenly  or  gradually  subjecting  it. 

The  means  which  the  plant  avails  itself  of  are  seldom 
abrupt  and  violent,  like  the  manifestation  of  muscular  con- 
tractility, but  more  frequently  take  the  form  of  the  modi- 
fication of  some  rhythm  which  is  characteristic  of  its 
behaviour.  A  few  cases  of  sudden  and  sharp  change  are 
met  with,  as  when  the  leaf  of  Mimosa  droops  on  being 
touched,  or  when  that  of  Dioncea  rapidly  closes  over  its 


378  VEGETABLE  PHYSIOLOGY 

captured  prey.  Less  conspicuously  purposeful  are  those 
changes  in  metabolism  which  are  brought  about  in  conse- 
quence of  interference  with  the  supply  of  food  or  oxygen,  but 
even  here  evidence  of  purpose  can  be  found  if  sought  for. 

To  understand  the  purposeful  changes  in  the  behaviour 
of  plants  when  they  encounter  modification  of  their  sur- 
rounding conditions,  we  may  consider  briefly  the  nature  of 
their  environment.  In  the  case  of  an  ordinary  terrestrial 
plant  we  find  it  to  be  as  follows  :  The  root  system  is 
embedded  in  the  soil,  among  the  particles  of  which  the 
young  root  branches  ramify  as  they  grow,  and  to  them  the 
root-hairs  become  firmly  attached ;  the  soil  undergoes 
usually  only  comparatively  small  changes  of  temperature, 
but  is  subject  to  a  great  deal  of  variation  with  respect  to 
the  amount  of  water  it  contains  and  the  distribution  of 
that  water  ;  it  is  composed  of  various  materials,  partly 
organic,  partly  inorganic,  many  of  which  are  of  great 
advantage  to  the  plant,  but  others  of  them  are  of  no  use  to 
it ;  of  the  former,  some,  though  valuable,  are  not  in  a  suit- 
able condition  for  absorption.  The  stem  rises  vertically  into 
the  air  and  bears  its  branches  and  leaves  ;  the  air  sur- 
rounding them  contains  a  varying  amount  of  aqueous 
vapour,  together  with  a  fairly  constant  quantity  of  carbon 
dioxide.  The  subaerial  portion  is  subjected  to  the  alterna- 
tion of  day  and  night,  involving  almost  continuous  changes 
of  degree  of  illumination,  together  with  varying  direction 
of  the  incident  rays.  During  these  times  it  meets  with 
considerable  variations  of  temperature  and  moisture  as 
well  as  light.  The  whole  plant  is  constantly  acted  on  by 
the  force  of  gravity.  The  subterranean  portions  are  less 
affected  by  light,  but  they  nevertheless  receive  a  certain 
amount  through  the  crevices  between  the  particles  of  the 
soil,  which  varies  from  time  to  time  both  in  amount  and 
in  direction.  The  environment,  though  to  a  certain  extent 
constant,  is  nevertheless  continually  varying  in  these 
respects,  so  that  no  two  plants  are  situated  exactly  simi- 
larly, though  they  may  be  growing  side  by  side. 


STIMULATION  AND  ITS  EESULTS  379 

The  surroundings  of  an  aquatic  plant,  though  in  some 
respects  very  different  from  those  of  a  terrestrial  one, 
exhibit  the  same  general  features  and  are  subject  to  almost 
as  frequent  disturbances,  though  a  watery  environment  is 
more  uniform  than  a  terrestrial  one. 

We  have  considered  already  the  effects  which  are  pro- 
duced by  extremes  of  light  and  darkness  upon  the 
behaviour  and  the  structure  of  plants.  We  have,  however, 
still  to  examine  the  rhythmic  excitations  to  which  plants 
are  subjected  by  the  variations  of  illumination  which 
accompany  the  alternation  of  day  and  night.  These  are  not 
accompanied  in  every  case  by  conspicuous  responses  which 
can  be  easily  observed ;  but  certain  plants  exhibit  a  some- 
what curious  behaviour  under  these  conditions.  This  is 
especially  connected  with  the  positions  of  their  leaves,  which 
assume  different  positions  during  the  day  and  the  night. 
This  sensitiveness  to  the  alternation  of  light  and  darkness 
is  not,  however,  confined  to  ordinary  foliage  leaves,  but  is 
in  many  cases  shared  by  cotyledons  also.  The  degree  of 
sensitiveness  varies  greatly  in  different  plants. 

This  form  of  irritability  is  manifested  in  a  very  marked 
degree  by  many  plants  of  the  Leguminosce,  the  Oxalidacece, 
and  a  few  other  Natural  Orders.  Mimosa  pudica  may  be 
mentioned  as  especially  favourable  for  examination  in  this 
particular.  When  this  plant  is  removed  from  light  to 
darkness  ilb  leaflets  droop,  and  the  opposite  pairs  become 
closely  approximated  to  one  another,  so  that  their  upper 
surfaces  are  in  contact.  On  being  restored  to  light  they 
separate  again  and  attain  their  former  expanded  condition, 
but  little  time  intervening  before  the  change  of  position  is 
assumed  in  either  case.  Another  very  good  instance  is 
afforded  by  Desmodium  gyrans,  the  so-called  Telegraph 
plant,  the  rhythmic  movements  of  whose  lateral  leaflets 
have  already  been  spoken  of.  During  the  day  its  leaves 
are  extended  almost  at  right  angles  to  the  stem  (fig.  153,  A)  ; 
as  night  draws  on,  the  terminal  leaflets  droop  till  they 
assume  a  position  almost  or  quite  parallel  to  the  stem 


380 


VEGETABLE  PHYSIOLOGY 


(fig.  153,  B).  The  leaves  of  many  others  take  up  still  more 
curious  positions,  in  some  cases  becoming  twisted  on  their 
petioles,  or  folded  together  in  various  ways.  In  some,  as 
in  Nicotiana  glauca  (fig.  154),  they  rise  instead  of  falling 
and  become  somewhat  closely  approximated  to  each  other. 
These  changes  of  position  are  generally  spoken  of  as 
nyctitropic  or  sleep  movements,  though  the  latter  term  is 
misleading  if  it  be  interpreted  to  mean  a  sleep  similar  to 


FIG.  163, — Desmodium  gyrans.     (After  Darwin.) 

A,  stem  with  leaves  as  seen  during  the  day;    B,  a  similar  stem  with  leaves  in 
the  nocturnal  position,  pointing  downwards. 

that  of  animals.  The  latter  phenomenon  is  attended  by  a 
temporary  suspension  of  sensitiveness  and  a  diminution  of 
rigidity,  which  is  not  necessarily  the  case  with  the  move- 
ments which  we  are  discussing. 

It  is  not  difficult  to  prove  that  these  curious  changes  of 
position  are  effected  in  response  to  the  stimulation  of  the 
alternation  of  light  and  darkness,  or  to  a  rhythmic  differ- 
ence in  the  amount  of  light  which  they  receive.  The 
accompanying  rhythmic  variation  of  temperature  no  doubt 
in  some  cases  also  plays  a  part  in  the  stimulation. 


STIMULATION  AND  ITS  EESULTS 


381 


If  a  plant  which  changes  the  position  of  its  leaves  as 
described  is  placed  for  a  time  under  constant  conditions 
such  as  darkness,  the  periodic  movement  is  soon  very 
much  interfered  with,  even  before  the  effect  of  darkness  is 
evident  in  the  loss  of  tone.  If  the  rhythmic  stimulus 
is  not  regularly  applied  the  movement  ultimately  stops. 
The  cessation  is  not,  however,  abrupt,  but  with  most  plants 
the  movements  will  continue  for  at  least  a  day.  The 


FIG.  15^—Nicotiana  glauca.     (After  Darwin.) 

A,  shoots  with  leaves  explanded  during  the  day  ;  B,  the  same  in  the 
nocturnal  position. 


rhythm  of  the  nyctitropic  movement  is  excited  by  the 
stimulus,  and  is  dependent  for  its  permanency  upon  the 
continuation  of  the  stimulating  changes.  Plants  which 
are  found  in  other  countries  to  show  this  sensibility  will, 
when  cultivated  in  England,  perform  the  movements  at 
the  normal  hours,  and  not  at  times  corresponding  to  the 
occurrence  of  day  and  night  in  the  countries  from  which 
they  come.  Nor  is  it  the  mere  alternation  of  day  and 
night  which  they  appreciate ;  it  is  rather  the  difference 
between  the  illumination  they  receive  during  the  two 


382  VEGETABLE  PHYSIOLOGY 

periods  which  constitutes  the  stimulus,  for  some  of  them 
will  not  assume  the  nocturnal  position  unless  they  have 
been  brilliantly  illuminated  during  the  day.  The  degree 
of  sensitiveness  in  this  case  is  not  so  great  as  in  those  where 
the  diurnal  and  nocturnal  positions  are  always  regularly 
assumed. 

The  peculiar  movements  which  the  leaves  perform  in 
response  to  this  stimulus  are  brought  about  by  different 
-mechanisms  in  different  cases.  In  young  leaves  they  are 
attendant  upon  growth,  and  are  brought  about  by  varia- 
tions of  turgescence  upon  the  two  sides  of  the  leaf  or  its 
petiole,  which  are  frequently  followed  by  growth.  We 
have  seen  that  during  growth  the  internal  turgescence 
varies  rhythmically,  and  leads  to  the  curious  movements 
of  nutation  or  circumnutation.  The  actual  nyctitropic 
movement  is  in  these  cases  a  modification  of  the  extent  of 
the  circumnutation,  the  original  rhythm  being  affected  by 
the  stimulus.  The  leaves  which  exhibit  it  can  be  seen 
by  careful  observation  to  be  circumnutating  during  the 
day.  When  they  assume  their  nocturnal  position  it  is 
generally  effected  by  their  describing  a  much  longer  ellipse 
than  that  of  their  ordinary  movement.  In  some  cases 
only  a  single  ellipse  is  described  during  the  twenty-four 
hours  ;  in  others  two  ellipses,  the  nyctitropic  one  being  much 
the  greater  in  amplitude.  In  yet  other  cases,  several 
ellipses  may  be  described  in  the  same  time. 

Adult  leaves  which  show  this  movement  do  so  by  virtue  of 
a  special  pulvinus,  a  kind  of  motile  organ  which  is  developed 
at  that  part  of  the  leaf -stalk  which  joins  the  stem.  This 
structure  has  special  developments  of  parenchyma  on  its 
upper  and  lower  sides  (fig.  155),  which  become  alternately 
turgid,  and  cause  the  leaf  to  droop  and  to  rise  accordingly. 
These  leaves  generally  exhibit  the  movement  for  a  much 
longer  period  than  those  in  which  it  is  brought  about  by 
variations  of  turgescence  accompanying  or  preceding 
growth.  This  naturally  follows  from  the  fact  that  the 
growth  of  leaves  is  not  as  a  rule  very  prolonged. 


STIMULATION  AND  ITS  RESULTS 


388 


That  these  movements  are  essentially  dependent  on 
the  power  of  the  protoplasm  to  receive  impressions  from 
without,  or  in  other  words  upon  its  possession  of  tone,  can 
be  seen  from  a  study  of  the  conditions  under  which  they 
are  performed.  When  the  soil  is  too  dry,  or  when  from 


FIG.  155. — PULVINUS  OF  Mimosa. 

a,  b,  the  succulent  parenchyma  of  its  upper  and  lower  sides ;    c,  bud ; 
d,  parenchyma  of  stem  ;    e,  pith. 


any  other  cause  the  protoplasm  in  the  cells  is  not  supplied 
with  water  in  sufficient  quantity,  they  cease.  When  the 
temperature  is  too  low  they  are  interfered  with.  Violent 
disturbance  of  the  protoplasm  by  shaking  the  plant  will  in 
some  cases  prevent  their  occurrence  for  one  or  two  nights. 

The  purpose  of  the  movement  is  somewhat  obscure  ;    it 
frequently  serves  to  protect  the  delicate  leaves  from  excessive 


384  VEGETABLE  PHYSIOLOGY 

radiation,  which  affects  them  very  prejudicially.  Their  upper 
surfaces  are  especially  liable  to  be  injured  in  this  way,  and 
it  is  noteworthy  that  in  all  cases  these  surfaces  are  most 
sheltered  when  they  take  up  their  nocturnal  positions. 
Often  the  upper  surfaces  of  leaflets  are  then  closely 
approximated  together  ;  in  Bauhinia  the  leaf  folds  itself 
upon  its  mid-rib  as  an  axis,  so  as  to  hide  completely  the 
ventral  face.  In  cases  of  such  complex  changes  of  position 
there  necessarily  must  be  a  very  delicate  co-ordination  of 
the  stimulation  received  by  so  many  cells  in  different  parts 
of  the  organ  concerned,  each  contributing  some  small  part 
of  the  movement  of  the  whole. 

The  nocturnal  movement  may  be  merely  a  relaxation  of 
the  effort  involved  in  maintaining  the  strain  of  tone  induced 
by  the  light  of  daytime,  in  a  certain  sense  implying  a  con- 
dition of  rest. 

Movements  which  bear  a  striking  superficial  resem- 
blance to  the  nyctitropic  movements  of  leaves  are  those  of 
the  opening  and  closing  of  certain  flowers,  which  take  place 
with  astonishing  regularity  and  precision  at  certain  hours 
of  the  morning  and  evening.  Though  they  seem  to  be 
influenced  by  the  alternation  of  light  and  darkness,  it  is 
more  probable  that  they  are  really  stimulated  by  the  changes 
of  temperature  which  accompany  such  alternation.  These 
variations,  to  be  effective,  must  lie,  however,  within  the 
range  already  indicated  as  being  necessary  for  the  mani- 
festation of  irritability  at  all.  The  movement  is  due  to 
rhythmically  varying  turgescence  of  the  cells  upon  the 
two  faces  of  the  growing  zone  of  the  floral  leaves,  which  is 
in  these  cases  a  narrow  transverse  band  situated  near  their 
bases.  This  change  of  the  turgescence  is  followed  in  many 
cases  by  actual  growth,  and  as  the  latter  is  not  of  prolonged 
duration  the  flower  can  only  open  and  close  a  few  times 
while  it  is  attaining  its  maturity. 

Besides  the  general  reactions  of  protoplasm  to  varia- 
tions in  those  features  of  the  environment  which  bring 
about  modifications  of  its  general  tone,  and  which  thus 


STIMULATION  AND  ITS  KESULTS  385 

affect  more  or  less  the  whole  plant,  we  find  instances  of 
special  sensitiveness  in  various  parts  to  influences  which 
are  not  appreciated  by  the  whole  of  the  living  substance, 
but  are  especially  received  by  the  young  growing  regions. 
Of  these  the  most  prominent  are  lateral  light,  gravity,  contact 
ivith  foreign  bodies,  moisture,  and  certain  chemical  stimuli. 
The  effects  produced,  however,  must  be  regarded  in  all 
these  cases  as  the  action  of  the  whole  organism,  which 
modifies  and  co-ordinates  the  behaviour  of  its  several  parts 
for  the  good  of  the  plant.  The  usual  form  of  response 
consists  in  various  modifications  of  local  growth. 

One  or  two  other  cases  of  special  sensitiveness  affecting 
only  particular  organisms  may  also  be  discussed. 

LATERAL  LIGHT. — The  effect  of  the  lateral  incidence  of 
light  may  be  studied  very  easily  in  the  case  of  young  seed- 
lings. When  one  of  these  is  so  placed  that  one  side  of  its 
stem  is  more  brightly  illuminated  than  the  opposite,  a 
curvature  soon  appears  in  the  part  which  is  actively  growing. 
This  is  of  such  a  nature,  and  takes  place  to  such  an  extent, 
as  to  cause  the  axis  of  the  plant  to  take  up  a  position  in 
which  it  is  parallel  to  the  direction  of  the  incident  rays. 
It  manifests  itself  in  some  cases  very  rapidly,  in  others 
more  slowly.  This  response  to  the  stimulus  of  a  lateral 
illumination  is  not  confined  to  the  stems  of  seedlings,  but 
may  be  seen  to  a  greater  or  less  degree  in  many  adult  plants. 
It  is  a  matter  of  common  observation  that  geraniums  grown 
in  a  window  all  bend  their  stems  and  petioles  towards 
the  illuminated  side. 

In  other  cases  the  same  stimulus  may  produce  an  opposite 
effect.  When  certain  young  roots  are  exposed  to  it,  they 
curve  so  as  to  place  themselves  in  the  same  position  with 
regard  to  the  incident  rays,  but  with  their  growing  apices 
in  the  opposite  direction.  Steins  are  said  accordingly  to 
grow  towards,  and  roots  away  from,  the  light-source.  This 
behaviour  is  not,  however,  confined  to  roots  ;  it  is  exhibited 
by  the  tendrils  of  Bignonia  capreolata,  the  peduncles  of 
Cyclamen  persicum,  and  by  many  other  organs. 

25 


386  VEGETABLE  PHYSIOLOGY 

Leaves  in  many  cases  show  a  similar  sensitiveness,  but 
the  position  they  assume  is  different  again.  They  place 
themselves  so  as  to  present  their  upper  surfaces  at  right 
angles  to  the  incident  rays. 

These  phenomena,  thus  associated  with  the  incidence 
of  a  lateral  light,  are  spoken  of  as  heliotropism,  aphelio- 
tropism,  and  diaheliotropism  respectively.  Sometimes  the 
terms  positive,  negative,  and  transverse  heliotropism  are 
employed.  The  purposeful  character  of  the  response  is 
generally  obvious  ;  the  heliotropism  of  a  stem  places  its 
leaves  in  the  most  favourable  position  for  the  action  of  the 
chlorophyll  in  the  process  of  photosynthesis  of  carbohydrate 
material ;  the  same  object  is  secured  by  the  diaheliotropism 
of  such  leaves  as  exhibit  it ;  the  apheliotropism  of  a  root 
assists  it  in  penetrating  into  the  crevices  of  the  soil.  The 
tendrils  of  Bignonia  are  aided  by  it  in  coming  into  contact 
with  a  support  about  which  they  can  twine.  The  aphelio- 
tropism of  the  peduncles  of  Cyclamen,  which  are  bent  down- 
wards in  a  hooked  fashion,  enables  them  to  grow  towards 
the  soil,  into  which  they  press  the  capsule,  thus  burying  the 


The  actual  stimulus  appreciated  by  a  stem  appears  to 
be  the  difference  of  the- illumination  on  the  two  sides  of  the 
organ  turned  towards  and  away  from  the  light.  It  seems 
as  if  the  plant  has  the  power  of  comparing  such  intensities 
of  illumination. 

The  response  to  the  stimulus  varies  sometimes  with  the 
age  of  the  organ.  The  hypocotyl  of  the  Ivy  is  heliotropic 
when  young,  but  becomes  apheliotropic  when  old. 

The  degree  of  sensitiveness  varies  very  greatly  in  different 
organs.  Some  of  the  seedlings  of  Phalaris  examined  by 
Darwin  responded  to  a  degree  of  illumination  so  feeble  that 
it  was  hardly  sufficient  to  cast  the  shadow  of  a  pencil  upon 
a  piece  of  white  paper  held  close  behind  it.  The  rapidity 
of  the  response  also  varies,  some  organs  bending  almost 
immediately,  while  others  do  so  much  more  slowly.  To 
this  point  we  shall  return  later.  The  movement  of 


STIMULATION  AND  ITS  KESULTS  387 

apheliotropism  is  usually  much  slower  than  that  of  helio- 
tropism. 

The  bending  is  not  caused  by  a  direct  interference  of 
the  light  with  the  part  actually  growing,  but  by  a  modifica- 
tion of  growth  caused  by  the  stimulated  protoplasm,  the 
nature  and  extent  of  the  alteration  being  conditioned  by 
the  intensity  of  the  stimulation.  It  would  seem  at  first 
as  if  the  retarding  effect  of  light  upon  growth  might  explain 
the  bending  of  the  organ  towards  the  light-source,  the 
non-illuminated  side  continuing  to  grow  and  the  illuminated 
one  being  prevented  from  doing  so.  This  explanation  is 
directly  contradicted  by  the  phenomenon  of  apheliotropism. 
It  is  moreover  proved  to  be  an  insufficient  explanation  by 
the  fact  that  the  part  which  is  sensitive  to  the  stimulus  is 
not  the  part  which  actually  bends.  Darwin  showed  this  by 
preventing  the  access  of  the  light  to  a  small  region  about 
one-tenth  of  an  inch  in  length  close  to  the  tip  of  the  seedling, 
when  he  found  that  the  heliotropic  curvature  did  not  take 
place,  although  the  normally  bending  part  was  illuminated. 
Further,  when  the  region  normally  curving  under  the 
influence  of  the  stimulation  is  mechanically  hindered  from 
bending,  the  curvature  takes  place  at  a  part  a  little  lower 
down,  which  normally  remains  straight. 

The  curvature  is  produced  by  an  acceleration  of  the  rate 
of  growth  on  the  convex  side,  together  with  diminished 
growth  on  that  which  becomes  concave.  When  the  lateral 
light  is  fairly  intense  the  resulting  movement  takes  place 
uninterruptedly  ;  when  it  is  only  weak  the  position  is 
assumed  by  a  series  of  zigzag  movements,  indicating  that  the 
new  movement  is  an  exaggeration  of  the  ordinary  cir- 
cumrmtatiori  of  the  part.  When  the  final  position  is  reached 
the  organ  is  found  to  circumnutate  about  the  new  direction 
of  the  axis. 

The  degree  of  the  intensity  of  the  lateral  light  is  a  very 
important  factor  in  the  stimulation.  While,  speaking 
generally,  stems  bend  towards  a  lateral  light  and  roots 
away  from  it,  a  great  increase  in  the  intensity  will  cause 

25* 


388  VEGETABLE  PHYSIOLOGY 

not  only  a  slowing  of  the  responsive  movement  of  the  stem, 
but  in  some  cases  an  actual  reversal  of  it.  Both  movements 
seem  to  have  for  their  object  the  placing  of  the  shoot  in  a  posi- 
tion most  favourable  to  the  plant  under  the  then  present  con- 
ditions of  illumination,  for  too  brilliant  a  light  is  deleterious. 
This  reversal  of  the  normal  curvature  in  the  presence  of  an 
excessive  stimulation  can  be  seen  with  great  clearness  in  the 
case  of  certain  sporangiophores  of  Phycomyces,  as  was  first 
shown  by  Oltmanns. 

A  somewhat  similar  response  to  the  influence  of  a  lateral 
light  is  exhibited  by  many  unicellular  organisms.  When 
these  are  exposed  to  oblique  illumination  they  take  up  a 
definite  position  with  regard  to  the  incident  rays,  placing 
their  long  axis  parallel  to  them  if  the  light  is  weak,  and  at 
right  angles  to  them  if  it  is  intense.  This  behaviour  is 
known  as  phototaxis  ;  it  is  exhibited  by  the  zoospores  of 
many  of  the  Algae  and  by  certain  Desmids. 

Before  leaving  the  subject  of  the  effect  of  a  lateral  light 
in  inducing  these  movements,  we  may  point  out  that  the 
phenomena  of  heliotropism  and  apheliotropism  must  be 
distinguished  from  those  of  photo-epinasty  and  photo - 
hyponasty,  which  were  alluded  to  in  the  last  chapter  (p.  371). 
The  difference  is  easily  seen,  for  in  the  latter  cases  the 
result  of  the  access  of  the  light  is  the  same,  whatever  be 
the  portion  of  the  organ  stimulated.  The  thallus  of  Mar- 
chantia  becomes  convex  on  the  dorsal  and  concave  on  the 
ventral  side,  whether  the  light  impinges  on  the  one  or  the 
other.  In  the  case  of  a  heliotropic  curvature  the  side 
which  is  stimulated  always  becomes  concave  ;  in  that  of 
an  apheliotropic  one  the  stimulated  side  becomes  convex. 

GRAVITATION. — The  force  of  gravitation  exerts  an  influence 
upon  plants  which  in  some  respects  resembles  that  of  lateral 
illumination.  Most  stems  grow  vertically  upwards  into  the 
air ;  primary  roots  grow  vertically  downwards  into  the 
soil.  A  few  organs,  among  which  may  be  mentioned  certain 
rhizomes  and  the  runners  of  many  plants,  grow  at  right 
angles  to  the  direction  of  gravity.  When  one  of  these  is 


T__J 


• 

390  VEGETABLE  PHYSIOLOGY 

placed  at  an  angle  from  the  position  which  it  usually  assumes, 
a  curvature  of  the  growing  organ  results,  which  lasts  till  the 
normal  attitude  is  regained.  When,  for  instance,  a  young 
seedling  is  detached  from  the  earth  and  laid  upon  its  side, 
the  stem  gradually  curves  through  an  angle  of  90°  and 
becomes  erect,  while  the  young  root  curves  in  the  opposite 
direction  till  it  points  vertically  downwards.  Similarly 
when  a  runner  is  placed  vertically,  its  apex  is  slowly  de- 
flected till  it  again  grows  parallel  with  the  soil.  These 
movements  are  termed  apogeotropic,  geotropic,  and  diageo- 
trdpic  respectively. 

To  prove  these  movements  to  be  responses  to  the  stimulus 
of  gravitation  it  is  necessary  to  eliminate  the  action  of  the 
latter  force,  and  to  observe  the  direction  of  growth  under 
the  new  conditions.  This  can  be  done  by  causing  the  plant 
to  grow  supported  upon  an  apparatus  known  as  a  Klinostat 
(fig.  156).  The  plant,  growing  in  a  flower-pot,  is  fixed  in  a 
wooden  box  B,  which  is  secured  by  a  thumb-screw  ih  to  the 
plate  pi;  the  box  is  cubical  in  form,  and  can  be  fixed  either 
as  shown  in  the  figure,  or  with  the  axis  of  the  pot  at  right 
angles  to  the  spindle  k  of  the  klinostat.  The  plate  pi  is 
attached  to  this  spindle,  which  ends  in  a  point  turning  in  the 
upper  end  of  the  left-hand  support  s.  The  spindle  is  also 
supported  at  g  on  the  friction  wheel  jr.  The  spindle  (with 
the  plant  attached)  is  made  to  rotate  by  means  of  a  band 
of  silk  dr,  passing  round  the  wheel  w,  and  also  round  a 
pulley  on  one  of  the  axles  of  an  American  watch-action 
clock  c,  which  is  attached  by  means  of  the  screw  E  to  the 
support  §'.  By  passing  the  driving-gear  over  the  large 
pulley  W ,  the  spindle  is  made  to  rotate  once  in  twenty 
minutes.  By  arranging  wheels  of  different  sizes  at  this 
point,  the  period  of  rotation  can  be  made  longer  or  shorter. 

When  the  plant  is  placed  in  a  horizontal  position  on  the 
revolving  plate,  every  face  of  its  axis  comes  successively 
under  the  influence  of  gravity,  so  that  all  parts  of  it  are 
affected  equally  and  similarly.  No  curvature  of  the  hori- 
zontal axis  of  the  plant  then  occurs  in  any  direction. 


CALIFORNIA,  COLLEGE 
of  PHARMACY 

STIMULATION  AND  ITS  KESULTS  391 

Another  experiment,  due  to  Knight,  pointing  to  the 
same  conclusion,  is  that  of  growing  a  plant,  preferably  a 
seedling,  upon  a  rapidly  revolving  wheel  mounted  on  a 
vertical  axis.  When  the  speed  of  the  revolution  is  suffi- 
ciently great,  though  the  plant  is  exposed  all  the  time  to 
the  action  of  gravitation,  the  centrifugal  force  of  the  appa- 
ratus is  so  much  greater  than  the  force  of  gravity  that  the 
plant  does  not  respond  to  the  latter.  Instead,  it  responds 
to  the  stimulus  of  the  rapid  rotation  or  centrifugal  force,  and 
the  root  grows  outwards  from  the  centre  of  the  wheel  while  the 
stem  grows  inwards  towards  it.  The  force  acts  much  like 
that  of  gravitation,  and  the  plant  responds  to  it  in  a  similar 
way,  the  root  growing  in  the  direction  of  the  force  and  the 
stem  in  one  opposite  to  it.  If  the  rotation  is  conducted  at 
less  speed,  so  that  the  centrifugal  force  is  about  equal  to 
that  of  gravitation,  the  position  assumed  by  the  axis  of  the 
plant  is  that  of  a  resultant  between  the  two  forces,  in  which 
it  makes  an  angle  of  about  45°  with  the  vertical. 

As  in  the  case  of  heliotropic  curvature,  the  part  which 
receives,  or  is  sensitive  to,  the  stimulus  is  not  the  part  which 
curves.  In  the  case  of  a  root  it  has  been  demonstrated  by 
Darwin,  and  more  recently  by  Pfeffer,  that  the  sensitive  part 
is  the  tip ;  while  the  curvature  takes  place  at  a  point  further 
back,  where  active  growth  is  taking  place.  The  curvature 
is  caused  by  a  similar  modification  of  the  growth  on  the 
two  sides  of  the  curving  organ. 

The  action  of  a  seedling  under  the  stimulus  of  gravity 
seems  to  indicate  that  the  plant  possesses  an  appreciation 
of  direction.  In  whatever  position  the  seedling  is  placed,  so 
long  as  it  is  free  to  grow  without  interference,  its  root  will 
grow  vertically  downwards,  executing  whatever  curvature 
is  necessary  for  it  to  attain  that  direction. 

The  movements  of  geotropism  and  apogeotropism  are 
not  confined  to  growing  organs.  When  the  haulm  of  a 
grass  is  placed  horizontally  on  the  ground,  as  is  the  case 
when  a  patch  of  wheat  or  other  cereal  is  beaten  down  by 
wind  or  storm,  after  a  time  it  again  becomes  erect.  The 


392  VEGETABLE  PHYSIOLOGY 

new  position  is  due  to  the  renewal  of  growth  on  the  under- 
sides of  the  swollen  nodes,  which  is  excited  by  the  stimulus 
and  proceeds  till  the  stem  is  again  vertical. 

The  way  in  which  gravitation  affects  the  sensitive  part 
of  the  root  is  obscure,  for  we  have  no  conception  of  the 
nature  of  the  force.  It  has  recently  been  suggested  that 
the  stimulation  is  brought  about  by  the  presence  of  movable 
starch  grains  in  the  cells  of  the  sensitive  area.  When  the 
root  is  pointing  downwards  these  grains  lie  on  the  front 
walls  ;  when  it  is  displaced  they  fail  to  be  symmetrically 
distributed  on  these,  and  may  impinge  on  the  lateral  walls 
that  should  be  vertical.  In  this  way  a  stimulus  due  to  the 
change  of  position  may  arise  from  such  unusual  contact 
with  the  movable  grains.  These,  which  may  include 
other  small  bodies  than  starch  grains,  have  been  called 
statolitUs. 

CONTACT  WITH  A  FOREIGN  BODY. — Many  instances  of 
sensitiveness  to  this  form  of  stimulus  have  been  observed. 
When  a  leaf  of  Mimosa  pudica  is  handled,  the  leaflets  all 
droop  downwards  with  great  suddenness,  and  if  the  handling 
is  very  rough,  all  the  leaves  on  the  plant  behave  similarly. 
When  a  stamen  of  Berberis  is  touched  at  a  point  a  little 
below  the  anther,  the  whole  stamen  bends  forward  towards 
the  pistil.  The  stigma  of  Mimulus,  which  is  composed  of 
two  lobes  normally  extending  outwards  from  each  other, 
will  close  if  either  lobe  is  touched  with  a  fine  point,  so  thai 
the  upper  surfaces  come  into  contact  with  each  other. 
When  an  insect  alights  on  the  surface  of  a  leaf  of  Drosera, 
the  tentacles  with  which  it  is  furnished  slowly  curl  over,  so 
that  their  terminal  glands  are  brought  together  at  the 
exact  point  of  irritation,  and  at  the  same  time  the  glands 
are  excited  to  pour  out  a  viscid,  slightly  acid,  secretion 
which  is  capable  of  digesting  the  proteins  of  the  insect's 
body.  The  leaf  of  Dioncea,  the  Venus's  fly-trap,  which  is 
normally  widely  expanded,  closes  with  some  rapidity  when 
a  touch  is  applied  to  one  of  the  six  sensitive  hairs  which 
spring  from  its  upper  surface.  The  leaf  closes  as  if  the 


STIMULATION  AND  ITS   EESULTS  393 

mid-rib  were  a  hinge,  bringing  together  the  upper  surfaces 
on  each  side  so  as  to  imprison  the  body  which  touches  it. 

This  form  of  sensitiveness  is  exhibited  in  a  very  striking 
way  by  the  growing  apex  of  a  young  root.  If  the  tip  of  a 
seedling  bean  is  stimulated  by  pressing  it  lightly  against 
some  hard  particle,  or  if  a  small  piece  of  cardboard  is  attached 
by  a  drop  of  water  or  very  dilute  gum  to  one  side  of  its 
apex,  a  curvature  speedily  results,  which  causes  the  root  to 
bend  away  from  the  irritating  body.  If  the  movement 
takes  the  sensitive  part  away  from  the  latter  the  curvature 
is  slight ;  but  if,  as  in  the  case  of  the  attached  cardboard, 
the  foreign  body  accompanies  it  in  its  displacement,  the 
curvature  will  continue  until  the  root  is  curved  completely 
round.  The  stimulus  in  the  case  of  this  movement  must  be 
prolonged,  differing  thus  from  the  cases  already  noted,  in 
which  a  mere  touch  is  sufficient  to  bring  it  about. 

Wounding  one  side  of  the  apex  of  the  root  by  bruising 
it,  or  applying  an  irritant  poison  such  as  lunar-caustic, 
brings  about  the  same  movement.  Indeed  such  wounding 
may  be  regarded  as  an  exaggeration  of  mere  contact. 

The  cause  of  this  curvature  must  be  the  sensitiveness  of 
the  protoplasm  to  the  stimulus  of  contact  or  of  injury. 
The  part  which  curves  is  some  little  distance  from  the  apex, 
at  which  the  capacity  for  receiving  the  stimulus  is  located, 
and  the  mechanism  of  the  curvature  is  a  modification  of  the 
distribution  of  turgescence  of  the  cells  in  the  zone  of  growth. 
It  is  only  while  that  part  is  actively  growing  that  the  curva- 
ture can  be  caused. 

Another  kind  of  curvature  can  be  detected  in  the  course 
of  the  growth  of  young  roots,  which  differs  fundamentally 
from  the  one  just  described,  and  the  two  must  be  carefully 
distinguished  from  each  other.  If  a  young  root  comes  into 
contact  with  an  obstacle  such  as  a  small  stone,  so  that  the 
latter  presses,  not  upon  the  tip  as  in  the  case  described,  but 
upon  the  region  of  the  growing  cells  some  little  distance 
farther  back,  a  curvature  results,  which  causes  the  root  to 
bend  towards  the  obstacle  instead  of  away  from  it.  This 


394  VEGETABLE  PHYSIOLOGY 

appears  to  be  due  to  the  contact  injuriously  affecting  the 
cells  which  are  pressed  upon,  so  that  their  growth  is  retarded 
or  stopped.  The  cells  on  the  other  side  of  the  root  not 
being  affected,  a  curvature  results  from  their  continued 
growth.  These  two  capacities  for  curvature  are  of  great 
assistance  to  a  root  during  its  growth  downwards  into  the 
soil.  On  coming  into  contact  with  a  particle  of  earth 
which  is  directly  opposed  to  its  progress,  the  tip  becomes 
"first  stimulated,  and  the  subsequent  curvature  causes  it  to 
be  deflected  past  the  obstacle  if  it  is  not  too  large.  A  little 
further  elongation,  followed  by  a  geotropic  movement, 
brings  the  growing  zone  into  contact  with  the  particle,  and 
the  converse  curvature  follows,  so  that  the  root  grows 
round  the  obstacle  and  then  resumes  its  normal  direction 
downwards,  under  the  stimulus  of  gravity. 

Perhaps  the  best  instance  of  sensitiveness  to  slight  contact 
is  afforded  by  the  behaviour  of  twining  organs,  tendrils, 
petioles,  and  climbing  stems,  the  twining  of  these  organs 
round  their  supports  being  altogether  due  to  it.  Very 
great  differences  of  irritability  are  met  with,  tendrils  generally 
possessing  it  in  a  very  high  degree,  but  climbing  stems 
often  exhibiting  it  very  feebly.  Indeed  some  observers 
deny  that  they  possess  this  form  of  sensitiveness.  In  the 
most  sensitive  cases  a  very  slight  touch  is  sufficient  to  bring 
about  a  perceptible  curvature  in  a  very  short  space  of  time. 
Darwin  found  that  the  contact  of  a  small  loop  of  thread, 
weighing  not  more  than  -gV  grain,  with  one  of  the  tendrils 
of  Passiflora  gracilis,  caused  it  to  bend,  while  a  mere  touch 
with  a  hard  substance  induced  it  to  assume  the  form  of  a 
helix  in  about  two  minutes.  This  is  perhaps  the  most 
sensitive  tendril  known  ;  with  others  a  stronger  stimulus 
is  needed,  and  the  time  taken  for  the  response  is  longer,  the 
irritability  varying  considerably.  Slight  rubbing  is  more 
effective  than  mere  contact. 

The  behaviour  of  tendrils  in  twining  is  somewhat  peculiar. 
When  young  they  are  continually  circumnutating,  and  if 
in  their  movement  they  come  into  contact  with  any  foreign 


STIMULATION  AND  ITS  KESULTS  395 

body,  they  begin  to  curve  round  it.  If  the  contact  is  not 
prolonged  the  tendril  will  curve  for  some  time,  but  will 
ultimately  straighten  itself  and  move  as  before,  till  it 
touches  something  else.  If,  on  the  other  hand,  the  body 
first  touched  is  one  round  which  the  tendril  can  twine,  it 
coils  itself  round  it ;  the  stimulus  thus  persists  and  the 
resulting  curvature  increases  it,  bringing  more  and  more  of 
the  sensitive  side  into  contact  with  the  support,  till  the 
latter  is  encircled  many  times  by  the  sensitive  twiner.  The 
coiling  is  seldom  confined  to  the  part  of  the  tendril  in  contact 
with  the  support,  but  the  free  part  between  the  latter  and 
the  axis  of  the  plant  also  twists  itself  into  a  kind  of  helix. 
If  the  two  are  not  very  close  together  this  helix  usually 
shows  two  parts,  the  coils  of  which  are  in  opposite  directions. 
This  is,  however,  only  because  the  filamentous  body  is 
attached  at  both  ends. 

The  sensitive  region  varies  in  different  tendrils,  but  it 
cannot  be  so  strictly  localised  as  in  the  case  of  a  growing 
root.  They  are  usually  irritable  on  one  side  only,  which 
is  slightly  concave,  though  in  some  cases  the  sensitiveness 
extends  all  round  them.  The  lower  part  of  a  tendril  is,  as 
a  rule,  only  sensitive  to  prolonged  contact.  Their  sus- 
ceptibility further  varies  with  their  age,  being  greatest 
when  they  are  about  three-parts  grown.  The  part  which 
first  responds  to  the  stimulus  is  usually  the  part  touched ; 
but,  as  we  have  seen,  the  coiling  also  takes  place  nearer  their 
bases,  so  that  we  have  an  evident  transmission  of  the 
stimulus  backwards,  as  in  other  cases  noted.  The  method 
of  response  is  usually  an  increase  of  turgidity  upon  the 
convex  side,  followed  by  greater  growth.  In  many  instances 
careful  measurements  have  shown  that  both  the  concave 
and  convex  parts  grow  during  the  coiling;  but  in  a  few 
cases  the  concave  side  either  does  not  grow  or  becomes 
actually  shorter  than  before. 

This  sensitiveness  to  contact  which  is  so  markedly  shown 
by  tendrils  is  possessed  also,  though  to  a  much  smaller 
extent,  by  most  climbing  stems.  These  organs  show  the 


396  VEGETABLE  PHYSIOLOGY 

movement  of  circumnutation  very  conspicuously,  the 
portion  which  takes  part  in  the  formation  of  the  spiral 
being  frequently  of  considerable  length.  This  is  of  course 
a  great  advantage  in  enabling  the  stem  to  find  a  support. 
The  continuation  of  the  circumnutating  movement  after 
contact  with  such  support  has  given  rise  to  the  view  that 
circumnutation  alone  will  enable  climbing  to  take  place. 
Consideration  of  the  behaviour  of  various  twining  stems 
with  supports  of  various  thicknesses  has  shown,  however,  that 
this  is  supplemented  by  changes  resulting  from  the  contact 
effected  by  circumnutation,  and  therefore  from  the  possession 
of  the  sensitiveness  under  consideration. 

Twining  stems  show  individual  peculiarities  in  the  direction 
of  their  twisting,  and  in  the  nature  and  particularly  the 
thickness  of  the  support  they  need.  The  stem  of  the  Hop 
twists  in  the  direction  taken  by  the  hands  of  a  watch  ;  that 
of  the  Convolvulus  in  one  diametrically  opposite.  The 
direction  of  the  twining  is  not,  however,  always  constant. 
Darwin  noticed  that  it  is  not  so  always  even  in  a  single 
individual.  In  Scyphanihus  elegans  it  is  reversed  in  succes- 
sive internodes  of  the  same  stem.  Many  of  our  ordinary 
climbers  can  twine  up  a  support  having  only  the  thick- 
ness of  a  piece  of  string  ;  other  plants,  particularly  the 
climbers  of  tropical  forests,  need  supports  of  some  inches  in 
diameter. 

The  twining  of  stems  is  often  accompanied  by  a  torsion 
of  the  stem,  or  a  twisting  round  its  own  axis.  This  is  not, 
however,  of  universal  occurrence. 

The  stimulus  of  contact  is  sometimes  followed  by  an  out- 
growth or  hypertrophy  of  the  part  affected.  This  is  seen 
in  the  tendrils  of  Ampelopsis  Veitchi,  which  on  prolonged 
stimulation  develop  little  adhesive  discs,  that  are  closely 
adpressed  to  roughnesses  in  the  surface  of  the  support  and, 
becoming  mechanically  attached  to  them,  enable  the  plant 
to  maintain  a  very  strong  hold  upon  the  wall  or  other 
support  to  which  it  is  clinging.  The  roots  of  TJiesium 
show  a  similar  property.  When  they  come  into  contact 


STIMULATION  AND  ITS  RESULTS  397 

with  other  roots  growing  near  them  they  develop  a  swell- 
ing at  the  point  of  contact,  from  which  certain  cells  grow 
out  and  penetrate  the  host,  forming  haustoria  (fig.  157). 
The  parasite  Cuscuta,  often  found  growing  on  clover,  is 
affected  in  the  same  way,  first  twining  round  the  clover 
stem  and  then  putting  out  haustoria,  which  penetrate  its 
tissues  (fig.  158). 

Another    form    of    irritability    is    exhibited     by    many 


Kiu.  157.—  Thesium  alpinum.    PIECE  OF  A  ROOT  WITH  SUCKEB  IN 
SECTION.      X     35.      (After  Kerner.) 

growing  shoots,  which  is  perhaps  somewhat  akin  to  sensi- 
tiveness to  contact.  It  is  an  appreciation  of  oscillation 
or  shaking.  If  a  shoot  is  gently  struck  laterally  several 
times  near  its  base,  its  apex  curves  over  towards  .the 
side  struck.  If  the  blows  are  given  near  the  apex,  the 
resulting  curvature  is  in  the  opposite  direction.  If  a  plant 
of  Mimosa  pudica  is  shaken,  the  leaves  fall  as-  they  do  when 
they  are  violently  handled. 

The  mechanism  whereby  the  response  to  the  stimulus  of 
contact  is  brought  about  "in  growing  organs  we  have  seen 
to  be  an  increased  turgidity  on  the  convex  side,  followed 
by  growth.  In  those  cases  where  the  organ  is  mature  it  is 


398 


VEGETABLE  PHYSIOLOGY 


evident  that  growth  can  have  nothing  to  do  with  the  move- 
ment.    In  these  instances  we  have  rather  to  do  with  a 


FIG.  158. — SECTION  OF  STEM  OF  DICOTYLEDONOUS  PLANT  ATTACKED  BY 
HAUSTOKIA  OF  Cuscuta. 


modification  of  turgescence,  involving  a  redistribution  of 
the  water  contained  in  the  organ.     The  falling  of  the  leaflets 


STIMULATION  AND  ITS  RESULTS 


399 


and  leaves  of  Mimosa  is  due  to  a  sudden  change  in  the 
protoplasm  of  the  cells  on  the  lower  sides  of  its  pulvini,  in 
consequence  of  which  water  escapes  from  them  into  the 
intercellular  spaces  between  them.  It  is  attended  by  a 
change  of  colour,  the  pulvinus  becoming  of  a  deeper  green 
in  consequence  of  the  replacement  of  the  air  there  by  water. 
If  a  leaf  is  cut  off  just  above  the  pulvinus  and  the  plant 


open  ;    2,  closed  : 
(  X 


FIG.  159. — LEAF  QJ?  Dioncea  muscipula. 

:  a,  lateral  view,  6,  surface  view ;  3.  one  of  the  sensitive  spines 
50) ;  4,  glands  on  the  surface  of  the  leaf  (  X    100). 


allowed  to  recover  from  the  effects  of  the  injury,  subsequent 
stimulation  of  an  adjacent  leaf  causes  water  to  exude  from 
the  cut  surface  of  the  pulvinus.  The  cases  of  the  irritable 
stamens  and  stigmas  are  probably  to  be  explained  similarly. 
The  closing  of  the  leaf  of  Dioncea  (fig.  159)  is  due  also  to  a 
redistribution  of  the  water  in  the  cells  of  a  band  of  tissue 
lying  along  the  mid-rib,  brought  about  by  a  rapid  change 
in  the  protoplasm,  perhaps  akin  to  contraction.  In  Drosera 
the  inflexion  of  the  tentacles  has  been  found  to  be  preceded 


400  VEGETABLE  PHYSIOLOGY 

by  a  peculiar  churning  movement  of  the  protoplasm  in  the 
cells  upon  the  side  which  becomes  concave.  This  move- 
ment, which  Darwin,  who  discovered  it,  called  aggregation, 
is  attended  by  a  loss  of  turgidity. 

MOISTURE. — Sensitiveness  to  variations  in  the  moisture 
of  the  environment  is  not  so  widely  distributed  as  are  the 
forms  of  irritability  hitherto  discussed.  It  is  exhibited 
among  green  plants  chiefly  by  young  roots  and  by  the 
Thizoids  of  the  Hepaticce ;  it  also  occurs  in  the  hyphaB  of 
certain  Fungi.  These  organs  tend  to  curve  in  the  direction 
of  a  moist  surface  if  they  are  growing  near  one.  When 
young  seedlings  are  cultivated  in  a  vessel  which  contains 
moist  sawdust  or  sand,  and  is  perforated  so  as  to  allow 
the  rootlets  to  protrude,  these  at  first  grow  vertically  down- 
wards, according  to  their  geotropism.  Soon  after  they 
protrude  they  curve  to  a  greater  or  less  extent  towards  the 
moist  surface,  as  if  seeking  the  moisture.  This  behaviour 
can  be  seen  more  easily  if  the  vessel  is  inclined  at  an  angle 
to  the  vertical.  The  phenomenon  is  known  as  Jiydrotropism. 
The  root-tip,  as  in  other  cases,  is  the  sensitive  part ;  while 
the  curvature  takes  place  further  back,  where  growth  is 
most  active.  Negative  hydrotropism  or  aphydrotropism  is 
very  rare,  being  exhibited  only  by  some  of  the  Myxomycetes, 
which  move  away  from  moisture. 

The  advantage  of  this  form  of  sensitivity  is  evident  in 
the  case  of  the  root,  which  by  virtue  of  it  is  drawn  towards 
the  moisture  of  the  soil  as  it  penetrates  between  its  particles. 

A  curious  instance  of  appreciation  of  lack  of  moisture 
is  afforded  by  Porlieria  hygrometrica,  which  under  such 
conditions  closes  its  leaflets  much  as  nyctitropic  plants  do 
when  light  gives  place  to  darkness. 

CHEMICAL  STIMULI. — We  have  already  alluded  to  the 
fact  that  the  various  metabolic  phenomena  of  plants  are 
influenced  very  considerably  by  changes  in  the  composition 
of  the  sap  which  the  cells  contain  ;  that  certain  constituents 
stimulate  the  protoplasts  to  initiate  or  to  alter  particular 
reactions  in  those  cells.  Besides  these  responses  to  chemical 


STIMULATION  AND  ITS  EESULTS  401 

stimuli  there  is  evidence  that  vegetable  protoplasm  can 
modify  its  normal  behaviour  in  other  ways  when  exposed 
to  similar  influences.  This  form  of  sensitiveness  is  less 
widely  distributed  than  those  which  we  have  just  discussed, 
but  instances  of  it  are  fairly  abundant,  especially  among 
the  more  lowly  forms  of  plants. 

A  certain  number  of  unicellular  organisms  are  strongly 
affected  by  the  presence  of  free  oxygen.  The  most  interest- 
ing case  of  this  sensitivity  is  that  of  Bacterium  termo. 
When  several  of  these  plants  are  placed  in  a  drop  of  water 
upon  a  slip  of  glass,  covered,  and  examined  under  the  micro- 
scope, they  are  found  to  collect  at  the  edge  of  the  cover- 
glass.  If  a  small  green  Alga  is  placed  in  the  drop  of  water 
with  them,  and  the  slide  exposed  to  light  of  sufficient 
intensity  to  enable  the  decomposition  of  carbon  dioxide  to 
take  place,  the  coincident  evolution  of  oxygen  attracts  the 
bacteria,  which  at  once  swarm  round  the  Alga.  So  sensi- 
tive are  they  to  this  attraction,  that  if  the  spectrum  of 
sunlight  is  thrown  upon  the  Alga,  the  bacteria  accumulate 
at  those  parts  which  are  illuminated  by  the  red  and  blue 
rays,  which  we  have  seen  to  be  capable  of  effecting  the 
exhalation  of  the  oxygen.  Kesponse  to  the  attraction  of 
oxygen  is  not  confined  to  these  bacteria  ;  it  is  exhibited  by 
many  zoospores  and  also  by  the  plasmodia  of  some  of  the 
Myxomycetes. 

When  the  necks  of  the  archegonia  of  the  BryopJiyta 
and  Pteridophyta  open  with  a  view  to  the  fertilisation  of 
the  oospheres  which  they  contain,  they  discharge  a  certain 
mucilaginous  fluid,  which  attracts  to  the  organ  the  free- 
swimming  antherozoids.  Careful  experiments  have  been 
made  in  many  cases  to  ascertain  what  is  the  nature  of  the 
attraction,  and  it  has  been  found  that  the  mucilage  contains 
various  substances  which  the  antherozoids  seek.  In  the 
cases  of  the  Ferns  and  some  Selaginellas,  it  has  been  deter- 
mined that .  the  attractive  body  is  malic  acid.  When  a 
capillary  tube  containing  a  weak  solution  of  this  substance 
is  inserted  into  water  containing  some  of  the  antherozoids, 

26 


402  VEGETABLE  PHYSIOLOGY 

they  make  their  way  very  quickly  to  the  orifice  of  the  tube. 
They  are  very  sensitive  to  the  presence  of  the  acid,  being 
guided  apparently  in  their  movements  by  very  slight 
differences  of  concentration.  When  the  acid  exceeds  a 
certain  strength  they  avoid  it  as  earnestly  as  they  seek  it 
when  it  is  in  greater  dilution. 

In  the  case  of  the  Mosses  the  attractive  substance    is 
cane-sugar.     Alkalies  in  any  degree  of  concentration  repel 
-the  antherozoids  of  both  groups. 

A  similar  sensitiveness  to  chemical  stimulation  marks 
the  plasmodia  of  the  Myxomycetes.  They  move  slowly 
towards  a  watery  extract  of  tan,  but  retreat  from  a  solution 
of  sugar,  glycerine,  or  certain  neutral  salts.  The  zoospores 
of  Saprolegnia  are  attracted  by  a  solution  of  extract  of 
meat. 

The  sensitive  tentacles  of  Drosera  can  respond  not  only 
to  contact,  as  already  described,  but  also  to  various  sub- 
stances placed  upon  the  leaf.  They  are  easily  induced  to 
bend  by  drops  of  liquid  containing  protein  matter,  such 
as  solution  of  albumin,  or  milk.  Certain  inorganic  salts, 
especially  carbonate  of  ammonia,  produce  the  same  effect. 
A  curious  instance  of  this  kind  of  irritability  has  been 
put  on  record  by  Miyoshi.  He  cultivated  certain  fungi  in 
gelatin  containing  a  small  proportion  of  sugar.  Under  the 
stratum  in  which  the  hyphae  were  ramifying,  he  placed 
another  containing  a  larger  proportion  of  sugar,  and  between 
the  two  arranged  a  membrane.  The  hyphse  very  soon 
grew  towards  the  stronger  sugar  solution,  and,  to  reach  it, 
penetrated  the  membrane. 

Other  instances  of  similar  behaviour  might  be  quoted. 
To  this  form  of  sensitiveness  the  name  of  chemotaxis  has 
been  given. 

A  few  other  forms  of  irritability  have  been  observed  in 
various  plants.  Certain  plants  growing  in  currents  of  water 
take  up  a  definite  position  with  regard  to  the  direction 
of  the  current,  some  growing  with  it,  others  against  it. 
Certain  plants  appreciate  small  differences  of  temperature 


STIMULATION  AND  ITS  RESULTS  403 

and  modify  their  growth  accordingly.  Almost  all  show  a 
peculiar  relationship  to  their  substratum,  stems  growing 
out  from  it  and  roots  into  it  in  a  direction  at  right  angles 
to  the  surface.  This  can  be  seen  by  cultivating  them  so 
that  they  do  not  emerge  in  the  normal  direction,  but  from 
the  side  of  a  cube  of  earth.  They  do  not  long  maintain 
this  direction,  as  they  speedily  feel  the  influence  of  light 
and  gravity.  If,  however,  appropriate  means  are  adopted 
to  eliminate  these,  the  growth  is  always  at  right  angles  to 
the  surface  of  the  soil  in  which  they  live. 

If  we  now  return  to  the  study  of  the  rhythmic  changes, 
which  we  have  seen  to  be  essentially  characteristic  of 
vegetable  protoplasm,  we  see  that  while  rhythm  is  no  doubt 
inherent  in  plants,  it  lends  itself  especially  to  such  changes 
as  are  caused  by  stimulation.  It  is  indeed  this  feature 
which  is  especially  brought  out  by  the  various  responses 
made  to  changes  in  the  environment.  While  it  occurs  with 
some  regularity  when  conditions  are  kept  constant,  it  is 
easily  affected  by  external  causes.  The  effect  of  continuous 
darkness,  or  of  too  great  cold,  or  other  abnormal  conditions, 
is  that  the  rhythmic  movements  are  made  irregular  and 
ultimately  stop.  In  many  cases  differences  in  the  degree  of 
illumination  during  the  day  affect  the  readiness  with  which 
the  nyctitropic  movements  of  the  leaves  are  brought  about. 
After  a  day  of  brilliant  sunshine  they  set  in  more  quickly 
than  after  one  of  dull  light. 

These  movements  may  show  indeed  a  secondary  induced 
rhythm  superposed  upon  a  normal  one.  The  movements 
of  heliotropism,  geotropism,  &c.,  may  be  looked  upon 
as  instances  of  this.  We  have  seen  that  they  are  based 
upon  the  ordinary  movement  of  circumnutation,  and  are 
in  fact  exaggerations  of  it.  As  the  latter  is  generally  a 
manifestation  of  a  rhythm  of  turgidity  in  the  cells  affected 
we  have  in  them  a  case  in  point.  In  other  cases  the  ten- 
dency to  rhythmic  change  can  be  demonstrated  by  the 
production  of  an  altogether  artificial  rhythm  induced  by 
submitting  the  plant  to  intermittent  stimulation.  F.  Darwin 


404  VEGETABLE  PHYSIOLOGY 

and  Pertz  have  described  a  very  interesting  experiment 
of  this  nature.  A  plant  was  fixed  to  a  spindle  placed 
horizontally,  on  a  modification  of  the  klinostat,  and  was  by 
an  arrangement  of  clockwork  made  to  undergo  a  semi- 
revolution  at  intervals  of  thirty  minutes.  The  force  of 
gravity  thus  exerted  its  effect  upon  alternate  sides  for  this 
interval  of  time,  so  that  each  side  of  the  stem  became  slightly 
convex  apogeo tropically  in  turn.  After  a  period  of  exposure 
.upon  the  instrument  the  clockwork  was  stopped.  Instead 
of  the  side  which  was  then  undermost  increasing  its  con- 
vexity till  the  stem  was  vertical,  the  two  sides  continued 
to  become  alternately  convex,  as  if  the  reversal  of  the 
instrument  was  still  taking  place.  There  was,  in  fact, 
an  artificially  induced  rhythm  manifested. 

While  the  movements  of  heliotropism  show  the  super- 
position of  an  induced  rhythm  upon  a  natural  one,  a  conflict 
between  the  two  can  be  observed  in  many  organs.  The 
heliotropic  curvature  is  not  brought  about  by  a  direct 
movement  of  the  bending  organ,  but  by  its  describing  a 
series  of  ellipses.  The  organ  at  the  time  of  the  incidence 
of  the  light  stimulus  is  performing  its  ordinary  circum- 
nutation,  the  apex  describing  a  circle.  The  effect  of  the 
stimulus  is  to  turn  that  circle  into  an  ellipse  ;  when  the 
rhythmic  impulse  coincides  with  the  stimulus  of  the  light, 
the  movement  is  accelerated  and  the  resulting  curve  takes 
the  form  of  the  greater  curvature  of  the  ellipse ;  when  the 
two  act  in  the  opposite  direction  to  each  other,  the  lesser 
curvature  of  the  same  figure  is  described.  The  same  result  is 
obtained  under  the  stimulus  of  gravity  when  the  stem  or 
root  has  by  any  means  been  inclined  from  the  vertical.  The 
ordinary  rhythm  of  circumnutation  is  resumed  when  the  new 
position  has  been  assumed  and  the  stimulus  consequently 
no  longer  acts. 

The  slow  response  to  the  action  of  a  stimulating  force 
may  frequently  be  explained  in  the  same  way.  Often, 
however,  the  long  delay  is  due  to  peculiarities  in  the  proto- 
plasm, which  will  be  discussed  in  the  next  chapter. 


STIMULATION  AND  ITS  EESULTS  405 

The  various  positions  which  are  assumed  by  the  different 
subaerial  organs  of  plants  are  evidently  those  in  which 
they  can  react  most  advantageously  with  their  environment. 
It  must  be  borne  in  mind,  however,  that  in  every  case 
during  natural  life  the  plant  is  receiving  coincidently  several 
kinds  of  stimulation,  the  effect  of  some  being  not  infre- 
quently antagonistic  to  that  of  others.  It  is  not  easy  to 
discriminate  between  these,  nor  to  say  how  the  influence  of 
each  helps  to  determine  the  resultant  response.  This  is  the 
more  difficult,  as  not  only  the  stimuli  themselves,  but  their 
relative  potencies1  differ  continually. 

Another  form  of  stimulation  differs  from  those  we  have 
discussed,  in  that  its  effect  becomes  evident  in  the  cells 
actually  stimulated.  This  is  the  stimulus  of  internal  pressure. 
When  the  central  cylinder  of  a  stem  begins  to  show 
secondary  thickening  a  strong  pressure  from  the  new 
vascular  tissue  sets  up  considerable  tension  in  the  outer 
layers  of  the  cortex,  and  tends  to  rupture  the  epidermis. 
This  strain  is  quickly  followed  by  the  appearance  of  a 
merismatic  layer,  the  cork  phellogen,  which  increases  the 
bulk  of  the  cortex  in  the  affected  area,  and  produces  as  well 
a  layer  of  cork. 

A  similar  cause  leads  to  the  appearance  of  the  inter- 
fascicular  cambium  across  the  medullary  rays.  The 
cambium  of  the  still  isolated  bundles  beginning  to  form 
new  wood  and  bast,  a  strain  of  the  medullary  ray  tissue 
lying  over  against  the  new  products  is  the  result.  This 
strain  is  responded  to  by  the  gradual  formation  of  meristern 
— the  interfascicular  cambium — which  slowly  extends 
across  the  ray  from  one  bundle  to  the  next. 

Another  form  of  it  is  seen  in  the  response  any  particular 
cell  makes  to  the  increase  of  its  own  turgor. 

The  infliction  of  a  wound  is  always  followed  by  an 
increased  growth  of  the  injured  tissues,  or  those  near  them. 


406  VEGETABLE  PHYSIOLOGY 


CHAPTER  XXIV 

THE  NERVOUS  MECHANISM  OF  PLANTS 

It  is  difficult  to  refrain  from  coming  to  the  conclusion,  from 
a  consideration  of  the  facts  which  have  been  discussed  in 
the  last  two  chapters,  that  the  nervous  system  of  the  animal 
kingdom  is  represented  in  the  vegetable  one.  That  plants 
are  sensitive  to  variations  in  the  conditions  surrounding 
them,  and  that  the  responses  they  make  to  such  variations 
are  purposeful  and  conduce  to  the  well-being  of  the  organism, 
is  abundantly  evident.  The  response  to  any  external 
stimulus,  moreover,  has  been  seen  to  be  dependent  upon 
the  plant  being  in  a  condition  of  tone — that  is,  of  health 
and  vigour.  If  its  well-being  has  been  interfered  with 
by  such  disturbances  as  deprivation  of  light,  or  lack  of 
oxygen,  or  exposure  to  too  high  or  too  low  a  temperature, 
no  response  is  given,  for  its  sensitiveness  is  in  abeyance  or 
destroyed.  The  lack  of  response  is  not  due  to  a  failure 
in  the  motor  mechanism  by  which  the  change  is  brought 
about,  but  by  an  absence  of  power  to  realise  the  altered  con- 
ditions which  would  constitute  a  stimulus  to  an  organism 
in  a  condition  of  full  health.  The  age  of  the  organism, 
again,  has  been  seen  to  have  an  important  influence  upon 
its  power  of  receiving  impressions  and  its  behaviour  in 
responding  to  them. 

We  have  already  called  attention  to  the  fact  that  the 
responses  made  to  stimuli  of  different  character  suggest 
definite  purpose.  No  reply  is  at  all  haphazard,  but  is 


THE  NERVOUS  MECHANISM  OP  PLANTS     407 

devoted  especially  to  some  definite  object  which  is  closely 
related  to  the  stimulus. 

Another  consideration  which  bears  upon  this  question 
is  that  an  extremely  small  stimulus  is  able  to  bring  about  a 
very  considerable  effect,  and  that  there  is  no  direct  or  simple 
ratio  between  the  intensity  of  the  stimulus  and  the  extent 
of  the  response,  whether  this  takes  the  form  of  movement 
or  chemical  change.  The  tendrils  of  Passiflora,  already 
alluded  to,  can  be  caused  to  move  by  the  contact  with 
them  of  a  small  piece  of  thread,  weighing  hot  more  than  ^V 
of  a  grain,  and  the  resulting  movement  will  be  of  consider- 
able extent  and  prolonged  for  some  time.  The  sensitive 
hair  of  the  leaf  of  Dioncea  needs  only  a  touch  to  cause  a 
rapid  movement  of  the  whole  leaf-blade  ;  the  pricking  of 
the  staminal  filament  of  Berberis  causes  a  considerable 
bending  of  a  relatively  bulky  body.  The  seedlings  of 
PJialaris  bend  with  some  speed  towards  a  light  which  is 
not  sufficient  to  cause  a  visible  shadow  at  the  distance  at 
which  they  are  placed  from  it. 

It  can  hardly  be  imagined  that  such  slight  disturbances 
can  act  mechanically  upon  the  parts  that  move.  This 
point  is  illustrated  by  an  observation  made  by  Wiesner, 
that  if  a  part  which  responds  only  to  the  stimulus  of  lateral 
light  is  exposed  for  some  time  to  such  an  illumination,  and 
then,  before  the  heliotropic  curvature  has  begun,  is  removed 
into  darkness,  it  will  slowly  bend  towards  the  side  which 
has  been  stimulated.  The  same  observation  has  been 
made  by  other  observers  in  the  case  of  the  stimulus  of 
gravitation.  There  is  no  explanation  possible  other  than 
that  the  stimulus  brings  about  changes  in  the  protoplasm 
of  the  cells  of  the  moving  part,  which  slowly  modify  their 
relation  to  the  water  of  their  contents,  so  that  a  great 
alteration  of  their  turgidity  results.  Moreover,  the  separa- 
tion of  the  part  stimulated  and  the  cells  which  are  the 
seat  of  the  resulting  action,  implies  that  there  must  be  in 
the  plant  a  means  of  more  or  less  rapidly  conducting  such 
external  impressions  from  one  part  to  another. 


408  VEGETABLE  PHYSIOLOGY 

If,  then,  we  adroit  that  there  is  even  a  rudimentary 
nervous  system  in  plants,  we  may  proceed  with  an  inquiry 
into  the  degree  of  its  differentiation,  and  the  completeness 
of  the  parallelism  which  it  may  be  expected  to  show  with 
the  corresponding  system  in  the  animal  kingdom. 

The  latter,  in  the  most  completely  organised  beings,  can 

be  shown  to  possess  certain  distinct  parts  :    one  by  means 

of  which  external  stimulation  is  received  and  appreciated  ; 

-another  whereby  movements,  &c.,  are  caused  ;    and  a  third 

which  is  a  regulating  and  controlling  part,  and  which  can 

co-ordinate  the  responses   to   stimulation,  or  can  initiate 

movements,   &c.,  in  its  absence.     There  are  also  definite 

paths  or  channels  by  which  the  three  are  brought  into 

connection  with  each  other,  generally  by  impulses  passing 

along   such   paths   in   definite    directions.     In   the   higher 

animals  these  are  well  differentiated  from  each  other  ;    we 

have  the  sense-organs,  each  devoted  to  and  fitted  for  the 

appreciation  of  particular  stimuli.     We  have  various  motor 

mechanisms,  usually  consisting  of  muscles  or  glands  which 

are  thrown  into  activity  in  consequence  of  the  reception  of 

impulses  by  sense-organs.     It  may  appear  to  be  straining 

matters  somewhat  to  class  these  as  part   of  the  nervous 

system,  but  it  does  not  appear  wrong  to  do  so  in  the  sense 

that  they  are  the  means  by  which  alone  the  working  of  the 

more  particularly  nervous  elements  of  that  system  can  be 

detected.     The  nervous  and  motor  systems  are  indeed  so 

closely  connected  that,  for  the  purposes  of  this  discussion, 

no  inconvenience  will  result  from  classing  them  together. 

In  the  animal  we  have  nerve-cells  occurring  singly  or  in 

groups,  forming  very  large  aggregations  such  as  the  brain, 

or  smaller  ones,  the  nerve  ganglia.     All  such  aggregations, 

or  even  single  cells,  are  concerned  in  the  task  of  co-ordinating 

stimuli  and  responses,  or  regulating  the  general  life  of  the 

organism.     Lastly,  we  have  well-differentiated  nerves  which 

serve  as  the  means  of  communicating  between  the  three 

other  factors  already  mentioned.     Each  nerve-fibre  ends  in 

one  direction  in  a  sense-organ  or  a  motor  mechanism,  such 


THE  NEEVOUS  MECHANISM  OF  PLANTS     409 

as  a  muscle  or  a  gland-cell,  and  in  the  other  in  a  nerve-cell 
belonging  to  the  co-ordinating  apparatus. 

We  can  easily  recognise  in  plants  certain  structures 
which  may  not  inaptly  be  termed  sense-organs,  as  we  can 
localise  in  them  the  power  of  perception  of  stimulating 
influences.  Darwin  found  that  the  seedlings  of  Phalaris 
were  not  sensitive  to  the  faint  light  employed  in  his  experi- 
ments, except  at  a  small  region  extending  about  -^  inch 
from  the  apex.  If  this  part  were  covered  by  an  opaque 
screen  in  the  shape  of  a  little  blackened  cap  of  not  sufficient 
weight  to  cause  any  flexion  of  the  stem,  the  seedlings  no 
longer  bent  towards  the  light.  The  tip  of  the  root  is  the 
only  part  which  is  sensitive  to  contact  in  such  a  way  as  to 
cause  the  growing  part  to  curve  so  as  to  carry  the  tip  away 
from  the  obstacle.  The  sensitiveness  of  any  particular 
cell  is  transitory,  passing  away  as  other  cells  are  formed 
in  front  of  it.  The  same  region  possesses  the  power  of 
appreciating  the  stimulus  of  gravitation.  This  has  been 
shown  by  Czapek  in  a  very  ingenious  manner.  He  caused 
the  roots  of  various  seedlings,  especially  Vicia  faba,  to 
grow  into  small  and  light  glass  tubes,  closed  at  one  end, 
and  bent  at  a  right  angle  about  TV  inch  from  that  end.  Tho 
cultivation  was  carried  on  on  a  klinostat  for  about  twelve 
hours,  when  the  root  had  penetrated  to  the  end  of  the  tube, 
and  had  consequently  become  sharply  bent  at  a  right  angle 
about  TV  inch  from  the  apex.  Eoots  so  prepared  were  then 
fixed  in  various  positions,  so  that  the  tip  was  rigid,  while 
the  growing  zone  just  behind  the  tip  was  free  to  curve. 
When  the  tip  was  vertical,  and  the  long  part  of  the  root 
horizontal,  the  root  continued  to  grow  without  any  curva- 
ture ;  when  these  conditions  were  reversed  a  geotropic 
curvature  resulted,  which  continued  as  long  as  the  tip  of  the 
root  was  mechanically  prevented  from  becoming  vertical. 
Other  observers  have  proved  the  same  thing  in  different 
ways.  Cisielski  amputated  the  tips  of  certain  rootlets,  and 
laid  them  horizontally  on  a  support.  They  did  not  then  show 
any  sensitiveness  to  gravitation  until  they  had  recovered 


410 


VEGETABLE  PHYSIOLOGY 


from  the  wound  and  a  new  root-tip  was  developed  upon  each. 
As  soon  as  the  new  tip  was  formed,  the  rootlets  showed  a 
power  of  reacting  to  the  stimulus  of  gravitation,  and  the 
curvature  resulted  in  the  usual  place.  If  we  turn  to  the 
reaction  of  the  leaf  of  Dioncea  to  contact,  we  find  that  the 
whole  leaf  may  be  somewhat  roughly  handled  without 
closing,  so  long  as  no  contact  is  made  with  the  hairs,  three 
in  number  (fig.  160),  which  arise  upon  a  particular  portion 


FIG.  160. — LEAF  OF  Dioncea  musclpula. 

1,  open  ;   2,  closed  :  a  lateral  view,  b,  surface  view;  3,  one  of  the  sensitive 
spines  (  X  50) ;  4,  glands  on  the  surface  of  the  leaf  (  X  100). 

of  the  blade.     So  soon,  however,  as  one  of  these  is  touched, 
the  leaf  closes. 

In  many  leaves  the  cells  of  the  upper  epidermis  are  convex 
on  their  upper  surfaces,  and  a  ray  of  light  passing  through 
them  is  brought  to  a  focus  somewhere  in  the  palisade  par- 
enchyma, with  such  distinctness  that  it  is  possible  to  use 
a  piece  of  detached  epidermis  in  the  fashion  of  a  number 
of  lenses  placed  side  by  side.  Haberlandt  has  attributed 
a  perceptive  function  to  these  cells,  which  he  calls  ocelli. 


THE  NEBVOUS  MECHANISM  OF  PLANTS     411 

There  is,  however,  some  doubt  as  to  how  far  this  interpre- 
tation is  justified. 

It  is  impossible  to  avoid  the  conclusion  that  we  have  to 
do  in  these  instances,  which  are  only  representative  ones, 
with  a  localisation  of  sensitiveness,  or  the  differentiation  of 
sense-organs.  If  we  compare  them  with  physiologically 
corresponding  regions  in  the  animal  we  find  a  certain 
agreement,  though  it  must  not  be  pressed  too  far.  The 
power  of  sight  is  very  complete  in  the  higher  animals, 
partly  in  consequence  of  the  highly  differentiated  character 
of  the  eye  ;  but  in  the  lower  animals  it  becomes  less  and 
less  perfect  as  we  descend  in  the  scale,  till  in  some  it  goes 
probably  little  further  than  the  power  of  appreciating  light. 
This  power  we  have  seen  to  be  possessed  by  certain  parts  of 
the  young  seedlings  of  various  plants  in  a  very  high  degree, 
and  by  other  organs  to  a  less  extent.  The  sense  of  touch 
may  be  compared  with  the  power  of  responding  to  the 
stimulus  of  contact  shown  by  tendrils  and  by  the  tips  of 
roots  ;  while  the  chemotactic  behaviour  of  the  organisms 
described  in  the  last  chapter  suggests  a  rudimentary  power 
of  taste  or  smell,  or  both. 

The  differentiation  of  these  mechanisms  in  plants  is 
from  an  anatomical  standpoint  very  slight.  Indeed,  no 
dissection  will  exhibit  any  special  feature  of  the  structure 
which  can  be  associated  certainly  with  the  perception  of 
the  stimulus.  It  is  a  property  of  the  protoplasm  of  the 
cells  in  question,  but  is  only  one  among  many  properties 
that  the  latter  possesses.  The  direction  of  differentiation 
in  vegetable  protoplasm  is  not  anatomical ;  but  such  a 
differentiation  is  very  considerable  physiologically.  The 
degree  of  sensitiveness  which  many  of  these  organs  possess 
is  extreme,  as  we  have  shown  already  by  several  examples. 

Another  somewhat  remarkable  fact,  in  view  of  the 
peculiar  character  of  the  differentiation  of  these  organs,  is 
that  the  same  sense-organ  is  sensitive  to  many  stimuli, 
though  in  different  degrees.  We  have  noticed  in  the  case 
of  the  root  that  its  tip  appreciates  contact,  gravitation,  and 


412  VEGETABLE  PHYSIOLOGY 

differences  in  hygrometric  condition,  though  nothing 
anatomical  suggests  such  diverse  powers.  If  such  a  sensi- 
tive organ  is  acted  upon  at  the  same  time  hy  two  stimuli 
which  usually  produce  opposite  movements,  the  resulting 
position  is  always  such  as  would  be  caused  by  the  greater 
influence  of  the  stronger  of  the  two.  The  organ  is,  in  fact, 
able  to  receive  both  stimulations  simultaneously,  and  to 
respond  to  each  as  if  the  other  were  not  received. 

If  we  turn  to  the  second  feature  of  the  nervous  system, 
we  find  that  the  motor  mechanism  of  the  plant  seems  at 
first  to  be  entirely  different  from  that  of  the  animal. 
Closer  consideration,  however,  lessens  the  difference  con- 
siderably. The  motor  mechanism  of  an  animal  is  nearly 
always  either  muscular  or  glandular.  The  vegetable 
cell  seems  to  have  much  more  in  common  with  the  gland 
cell  of  an  animal  than  with  its  muscles.  Stimulation  of  a 
nerve  going  to  a  gland  frequently  causes  a  flow  of  liquid  from 
the  latter,  probably  owing  to  a  change  in  the  permeability 
of  the  protoplasm  of  the  gland-cells.  The  contractile 
power  is  but  little  developed  in  vegetable  protoplasm,  and 
when  present  it  seems  to  be  rather  passive  than  active, 
to  be  associated  with  recoil  rather  than  true  contraction. 
Still,  the  latter  is  not  entirely  absent.  We  have  seen  that 
it  can  be  detected  in  the  pulsation  of  vacuoles,  in  ciliary 
motion,  and  in  the  crawling  movements  of  the  Myxomycetes. 

Though  the  power  of  contraction  is  comparatively 
seldom  found,  the  action  of  the  gland-cell  is  recalled  by  the 
power  which  vegetable  protoplasm  possesses  of  resisting  or 
assisting  the  transit  of  water.  The  effect  is  really  similar  in 
both  cases ;  in  the  animal  the  disturbance  to  the  protoplasm 
leads  to  a  contraction  of  its  substance,  in  the  plant  to  its 
lessening  its  resistance  to  the  passage  of  water  through 
it.  Each  protoplasm  responds  in  its  own  appropriate 
fashion,  which  is  based  upon  the  need  of  the  organism  of 
which  it  is  part.  The  main  requirement  of  most  animals 
is  freedom  of  locomotion  or  rapid  assumption  of  new  posi- 
tions by  the  body.  The  most  important  duty  of  the  plant  is 


THE  NEEVOUS  MECHANISM  OF  PLANTS      413 

the  regulation  of  the  water  supply  upon  which  its  con- 
stituent protoplasts  are  so  dependent. 

The  immediate  result  of  such  an  increase  of  perme- 
ability is  that  the  elastic  recoil  of  the  stretched  cell 
membranes,  which  we  have  seen  is  a  feature  of  every  turgid 
cell,  drives  some  of  the  water  out  of  the  cell,  causing  the 
latter  to  shrink  in  volume. 

The  effects  of  stimulation  may  be  seen  in  glandular 
organs  in  plants  as  well  as  animals.  Both  Drosera  and 
Dioncea  are  excited  by  contact  to  pour  out  on  to  the  surface 


FIG.  161, — CONTINUITY  OF  THE  PKOTOPLASM  OF  CONTIGUOUS  CELLS 
OF  THE  ENDOSPERM  OF  A  PALM  SEED  (Bentinckia).  Highly  magnified. 
(After  Gardiner.) 

a,  contracted  protoplasm  of  a  cell ;   6,  a  group  of  delicate  proto- 
plasmic filaments  passing  through  a  pit  in  the  cell-wall. 

of   their  leaves   acid   digestive   secretions,   which   are  the 
result  of  changes  in  the  activity  of  the  gland-cells. 

The  conduction  of  the  stimuli  received  is  due  in  the 
higher  animals  to  the  existence  of  differentiated  nerves. 
The  way  in  which  it  is  carried  out  by  plants  has  been  much 
debated,  but  since  the  discovery  of  the  continuity  of  the 
protoplasm  through  the  cell-walls  there  is  little  doubt  that 
we  have  here  a  similar  mechanism.  There  is  scarcely  any 
differentiation,  but  the  power  of  the  protoplasm  to  con- 
duct disturbances  from  one  part  of  the  cell  to  another  is  a 
matter  of  common  observation.  The  connecting  strands 
between  adjacent  cells  (fig.  161)  will  suffice  to  suggest  how 
impulses  from  the  tip  of  the  root  may  reach  the  growing  region. 


414  VEGETABLE  PHYSIOLOGY 

The  co-ordination  of  these  factors  we  have  seen  is  one 
of  the  most  marked  features  of  a  highly  differentiated 
nervous  system.  In  this  respect  we  cannot  note  anything 
in  the  plant  which  in  its  elaboration  or  in  its  peculiar 
efficiency  can  be  compared  with  the  co-ordinating  mechanism 
of  animals.  Certain  responses  to  stimulation  can  be  effected, 
but  no  definite  regulation  of  any  function  shows  any  great 
completeness.  We  have  seen  this  particularly  in  the  case 
of  the  influence  of  temperature.  Though  a  certain  range 
of  temperature  is  imperative  for  the  plant's  well-being,  it 
has  no  power,  or  but  little,  to  co-ordinate  its  own  produc- 
tion or  expenditure  of  heat  with  the  variations  of  tempera- 
ture to  which  it  is  exposed. 

Nor  can  we  observe  any  structural  differentiation  in  the 
direction  of  such  co-ordination,  though  very  complex  move- 
ments in  some  cases  point  to  its  existence.  We  may 
instance  the  assumption  of  the  nyctitropic  position  by  the 
leaves  of  Nicotiana  and  of  certain  Leguminosce.  The 
plant  shows  an  almost  complete  absence  of  the  differentiation 
which  reaches  its  highest  point  in  the  nerve-cell.  There 
is  apparently  no  co-ordinating  mechanism  which  receives 
the  impulses  from  the  sense-organs,  and  initiates  in  conse- 
quence the  resulting  movement.  One  case  only  has  so  far 
been  put  on  record  which  even  suggests  a  complexity  of 
this  kind.  Attention  has  been  called  by  Darwin  to  a 
peculiarity  in  the  behaviour  of  the  tentacles  of  Drosera, 
in  which  something  of  this  nature  is  seen.  When  one  of 
these  organs  is  stimulated,  its  actual  bending  is  preceded 
by  a  curious  motility  of  the  protoplasm  of  the  cells  of  its 
stalk  which  has  been  called  aggregation.  If  a  tentacle  on 
the  surface  of  the  leaf  is  excited,  the  tentacles  of  the  margin 
are  gradually  inflected  towards  the  excited  spot.  If  the 
cells  of  one  of  these  marginal  tentacles  are  watched  during 
the  experiment,  their  contents  are  found  to  undergo  this 
aggregation,  but  those  nearest  its  apex  manifest  it  first. 
If  the  aggregation  were  the  direct  effect  of  the  stimulus, 
those  which  it  reached  first,  i.e.  those  nearest  the  base  of 


THE  NEKVOUS  MECHANISM  OF  PLANTS     415 

the  tentacle,  would  respond  first.  The  stimulus,  apparently, 
has  to  travel  up  the  gland,  and  a  disturbance  has  to  originate 
at  its  apex  in  response,  this  disturbance  travelling  down  the 
tentacle  in  the  direction  of  its  base.  Darwin  has  pointed 
out  that  this  recalls  in  a  measure  the  reflex  action  of  the 
animal  organism. 

But    though    this    co-ordinating    power   is    very    feebly 
developed  we  cannot  deny  that  there  is  a  power  or  property 
of  protoplasm  which  represents  it,  even  if  in  only  rudi- 
mentary form.     We  have  already  alluded  to  the  purposeful 
character  of  the  responses  to  stimulation.     There  must  be 
some  means  by  which  an  appreciation  of  the  character  of 
the  stimulus  is  communicated  to  the  protoplasm,  which 
suggests  a  certain  possibility  of  perception  which  must  be 
the  antecedent  of  co-ordination.    We  do  not  know  whether 
the  fact  that  the  response  is  localised  depends  upon  the 
possession  of  particular  properties  by  the  responding  organ, 
so  that  while  the  impulses  set  up  in  the  sense-organ  travel 
in  all  directions  through  the  plant,  only  certain  cells  can 
be  excited  to  change  in  response  to  them,  or  whether  the 
paths  of  the  conduction  of  the  impulses  only  take  them 
to  the  responding  organ.     But  the  fact  remains  that  the 
response  bears  a  definite  relationship  to  the  stimulus,  par- 
ticularly to  its  locality,  and  in  a  less  degree  perhaps  to  its 
intensity.     If  a  root-tip  is  brought  into  contact  with  an 
obstacle,  the  bending  is  invariably  in  such  a  direction  as 
to  enable  the  root  to  pass  it.    When  one  is  allowed  to 
impinge  upon  a  small  stone  at  right  angles  to  its  direction 
of  growth,  the  curvature  continues  till  the  root  has  turned 
through  a  right  angle,  and  can  for  a  short  distance,  at 
any  rate,  grow  parallel  to  the  opposing  surface,  till,  passing 
it,  it   can  again  respond  to  the  influence  of  gravitation 
and  grow  vertically  downwards.     The  stimulus  causing  the 
movement  of  hydrotropism  serves  to  bring  the  root-hairs 
into  contact  with  the  moist  surface,  thus  enabling  them  to 
discharge  their  appropriate  function. 

The  behaviour  of  the  tentacles  of  Drosera  rotundifolia 


416  VEGETABLE  PHYSIOLOGY 

is  very  interesting  in  this  connection.  The  leaf  is  of  some 
size,  and  can  therefore  receive  stimuli  over  a  fairly  large 
area.  When  the  tentacles  bend  over  in  response  to  the 
alighting  of  an  insect,  they  do  not  do  so  irregularly,  but 
always  place  their  glandular  apices  directly  upon  the 
spot  which  is  the  centre  of  the  disturbance.  This  is  very 
definitely  purposeful,  the  invader  being  captured  and 
digested  wherever  it  alights,  as  all  the  tentacles  are  brought 
to  bear  upon  it. 

The  purposeful  character  of  heliotropic  and  diahelio- 
tropic  curvatures  is  also  very  evident,  the  leaves  being 
always  placed  thereby  in  the  position  most  favourable  to 
the  discharge  of  their  functions. 

The  very  rudimentary  differentiation  of  any  mechanism 
for  co-ordination  suggests  a  very  immobile  condition  of  the 
co-ordinating  protoplasm.  There  are  several  considerations 
which  support  this  view.  In  many  cases  the  movement 
of  heliotropism  does  not  commence  till  a  considerable  time 
after  the  access  of  the  lateral  light,  the  actual  time  varying 
in  different  cases.  Similarly  the  apogeotropic  curvature 
of  a  stem  placed  horizontally  may  not  be  observable  till 
the  stimulus  has  lasted  for  more  than  an  hour.  We  have 
what  is  generally  called  a  long  latent  period  before  the 
manifestation  of  the  irritability.  The  time  is  taken  up 
in  bringing  about  the  response  to  the  stimulus  and  not  in 
appreciating  it.  The  power  of  appreciation  is  generally 
rapid,  as  we  should  imagine  when  we  remember  the  great 
degree  of  sensitiveness  as  measured  by  the  smallness  of 
the  stimulus  which  is  necessary  to  produce  an  effect.  The 
sluggish  nature  of  the  co-ordinating  mechanism  can  be 
seen  from  the  fact  that  the  removal  of  a  stimulus  before 
any  response  to  it  has  become  evident  does  not  prevent 
that  response  from  subsequently  appearing.  If  young 
roots  are  laid  upon  their  sides  for  about  an  hour  and  a 
half,  and  their  tips  are  then  carefully  amputated  so  that 
they  no  longer  perceive  the  stimulus  of  gravitation,  they 
will  nevertheless  curve  after  a  while  towards  the  side 


THE  NERVOUS  MECHANISM  OF  PLANTS     417 

which  was  downwards  during  the  first  exposure.  The  same 
curvature  will  be  seen  if  they  are  placed  in  a  vertical  posi- 
tion after  the  amputation.  The  long  delay  in  the  response 
may  no  doubt  be  attributed  partly  to  the  disturbance  set 
up  by  the  amputation  ;  but  the  fact  that  the  response  to  the 
stimulus  does  eventually  take  place  shows  that  the  delay 
is  due  to  slowness  of  changes  in  the  responding  protoplasm 
and  not  in  the  part  which  is  sensitive. 

An  even  more  striking  instance  of  action  after  the  removal 
of  the  stimulus  which  has  originated  it— a  so-called  after- 
effect— may  be  seen  by  allowing  a  stimulus  to  operate  for 
some  time  and  then  reversing  its  direction.  This  can  be 
done  by  fastening  a  root  horizontally  in  a  damp  atmosphere 
and,  as  soon  as  the  curvature  commences,  inverting  it  so 
that  the  side  showing  the  slight  convexity  is  downwards. 
The  curvature  will  continue  in  the  original  direction  for 
some  time,  and  will  only  slowly  cease  and  be  replaced  by 
one  in  the  opposite  direction. 

We  can  distinguish  between  the  general  condition  of 
irritability,  or  the  state  of  tone,  and  these  special  forms  of 
sensitiveness  which  we  have  examined.  So  long  as  the 
conditions  remain  favourable  the  general  sensitiveness  of 
the  plant  is  maintained,  but  the  power  of  responding  to 
particular  impressions  may  disappear  from  various  causes 
without  any  disturbance  of  its  sensitivity  to  others.  The 
power  of  appreciating  differences  in  the  environment  varies 
with  the  age  of  the  plant,  disappearing  in  some  cases  from 
an  organ  while  it  still  retains  its  power  of  circumnutating. 
The  effect  of  a  prolonged  stimulation  is  sometimes  failure 
to  induce  a  movement.  In  the  case  of  Dioncea  this  is 
very  marked.  If  a  leaf  is  for  a  time  mechanically  pre- 
vented from  closing,  repeated  touching  of  one  of  the  sensi- 
tive hairs  brings  about  an  exhaustion  of  its  power  to  receive 
a  stimulus,  so  that  if  the  leaf  is  released  a  disturbance 
of  that  particular  hair  evokes  no  response.  At  first  it  may 
seem  doubtful  whether  or  no  the  interference  with  the 
free  responce  of  the  leaf  may  have  so  injured  the  motor 

27 


418  VEGETABLE  PHYSIOLOGY 

mechanism  as  to  make  it  incapable  of  acting.  The  exhaus- 
tion, however,  is  shown  to  be  that  of  the  hair  and  not  of 
the  blade  by  the  fact  that  touching  another  of  the  hairs  at 
once  causes  closure. 

The  nervous  sensitiveness  is  shown  by  this  and  many 
other  similar  experiments  to  be  capable  of  fatigue.  A 
similar  suspension  of  power  may  be  demonstrated  by 
exposing  the  sensitive  parts  to  anaesthetics,  such  as  the 
'vapour  of  chloroform  or  ether.  The  effect  of  these  drugs 
at  once  suggests  an  action  similar  to  that  which  they  have 
on  the  nervous  mechanism  of  an  animal.  When  the  effect 
of  the  fatigue  or  the  anaesthetic  has  passed  off,  the  organ 
again  becomes  capable  of  responding. 

While  we  are  able  from  these  considerations  to  recog- 
nise in  the  plant  a  nervous  system  in  some  way  com- 
parable to  that  of  an  animal,  we  must  clearly  recognise  the 
limitations  under  which  it  exists.  It  can  only  be  regarded 
as  rudimentary  and  as  showing  a  very  slight  degree  of 
differentiation.  This  we  have  seen  is  particularly  notice- 
able with  regard  to  its  co-ordinating  power.  Another 
feature  must  be  mentioned,  however,  before  leaving  the 
subject.  We  do  not  find  in  the  plant  any  indication  of 
anything  corresponding  to  the  higher  functions  of  the 
nervous  system  of  the  higher  animals.  There  is  little  evi- 
dence of  anything  which  we  may  compare  to  consciousness 
or  volition.  Though  many  of  the  responses  to  stimulation 
are  eminently  purposeful  we  cannot  regard  them  as  in  any 
way  modified  or  held  in  check  by  any  controlling  power. 
A  stimulus  will  produce  its  due  effect,  although  the  mani- 
festation of  that  effect  at  the  particular  moment  may  be 
followed  by  injurious  consequences.  The  connection  between 
the  sense-organ  and  the  motor  mechanism  is  apparently 
a  direct  one,  and  there  is  no  power  to  modify  it  possessed 
by  the  organism. 

Nor,  so  far  as  we  know,  have  we  in  plants  any  power 
of  initiative.  True,  there  are  many  movements  and 
changes  which  are  set  up  by  causes  that  have  their  origin 


THE  NEKVOUS  MECHANISM  OF  PLANTS     419 

in  some  alteration  of  the  protoplasm  which  we  cannot 
explain,  but  there  is  no  evidence  of  purpose  in  their  origina- 
tion. Even  the  locomotion  of  the  Myxomycetes  and  the 
Diatoms  shows  no  definite  purpose  except  when  it  is  clearly 
set  up  in  response  to  some  external  stimulus. 

Though  there  is  no  particular  differentiation  of  an  ana- 
tomical character  in  any  of  the  sense-organs  of  a  plant, 
there  is  nevertheless  a  differentiation  of  a  physiological 
nature  in  the  direction  of  sensitiveness  which  will  equal,  if 
not  surpass,  the  powers  of  the  sense-organs  of  an  animal. 
The  tendril  of  Passiflora  appreciates  and  responds  to  a 
pressure  which  cannot  be  detected  by  even  the  human 
tongue  ;  the  seedlings  of  Phalaris  readily  obey  the  stimulus 
of  an  amount  of  light  which  is  hardly  perceptible  by  the 
human  eye.  Many  plants  readily  detect  and  respond  to 
the  ultra-violet  rays  of  the  spectrum,  which  are  utterly 
invisible  to  man. 

The  extent  of  the  response  to  any  stimulus  is  of  course 
much  less  than  that  exhibited  by  an  animal ;  but  this,  as 
we  have  seen,  depends  upon  the  differences  in  the  motor 
mechanisms.  In  the  vegetable  protoplasm  we  have  a 
much  slower  response,  as  well  as  one  of  a  different  kind, 
the  effects  taking  as  a  rule  longer  before  they  are  fully 
manifested  and  lasting  for  a  longer  time  after  the  stimulus 
has  been  withdrawn.  We  have,  however,  as  in  the  animal 
mechanism,  a  much  better  response  to  a  cumulative  or 
prolonged  stimulation  than  to  one  which  is  rapid  and 
transitory. 


420  VEGETABLE  PHYSIOLOGY 


CHAPTER  XXV 

KEPKODUCTION 

The  phenomena  we  have  hitherto  been  considering  all 
concern  the  life  of  the  individual  plant.  As  this,  however, 
at  the  best  is  comparatively  limited  in  duration,  we  find  plants 
possessed  of  the  power  of  giving  rise  to  new  individuals.  The 
process  of  originating  each  new  individual  from  its  parent 
or  parents  is  known  as  reproduction. 

We  have  seen  that  the  life  of  the  plant  is  essentially 
bound  up  with  the  individuality  of  the  protoplasts  which 
compose  it.  Many  plants  consist  of  but  a  single  one  of 
these  organisms  ;  others  are  composed  of  many,  some  of  a 
very  large  number.  We  have  seen  reason  to  look  upon 
each  of  these  aggregations  of  protoplasts  as  a  large  colony 
whose  members  have  •  become  differentiated  in  various 
ways  to  carry  out  to  the  greatest  advantage  the  vital  pro- 
cesses of  all.  In  the  simplest  forms,  such  as  filaments  of 
protoplasts  like  Spirogyra  or  Ulothrix,  each  protoplast  is 
apparently  independent  in  its  behaviour,  though  mechani- 
cally attached  to  its  neighbours.  In  more  complex  and 
bulky  forms  this  independence  has  been  given  up  in  favour 
of  complete  co-operation  for  the  general  welfare. 

As  every  plant,  then,  is  composed  of  either  one  proto- 
plast or  many,  we  may  in  the  latter  case  distinguish 
between  the  colony  and  its  constituents.  The  term  in- 
dividual is  usually  associated  with  the  former,  and  we  speak 
of  reproduction  as  leading  to  the  appearance  of  such  in- 
dividuals without  making  any  reference  to  the  protoplasts 


KEPKODUCTION  421 

of  which  each  consists.  In  dealing  with  reproduction,  however, 
in  the  broad  sense  we  must  consider  also  the  development 
of  the  protoplasts  of  the  colony  as  well  as  of  the  appearance 
of  new  colonies  or  so-called  individuals.  Indeed,  in  the 
case  of  unicellular  plants  such  production  of  new  proto- 
plasts is  the  only  form  of  reproduction  possible. 

It  is  important,  however,  to  bear  in  mind  the  different 
individualities  of  the  protoplast  and  of  the  colony  of  which 
it  is  part.  In  a  filament  of  Ulothrix  or  other  thread-like 
alga,  each  protoplast  being  like  every  other  in  all  essential 
points,  we  may  regard  the  formation  of  new  protoplasts  in 
the  chain  as  a  process  of  reproduction  of  the  units.  As  the 
chain,  however,  grows  by  means  of  such  multiplication  of 
its  constituent  protoplasts,  and  has  a  distinct  individuality 
as  a  filament,  we  may  also  regard  the  process  of  multiplica- 
tion of  the  units  as  one  of  growth  in  the  length  of  the  chain. 
What  is  reproduction  of  the  units  of  construction,  the  proto- 
plasts, is  growth  of  the  individual,  the  colony.  The  same  thing 
is  seen  in  all  plants  which  consist  of  more  than  a  single  cell. 

We  may  study  the  method  of  multiplication  of  the 
protoplasts  either  in  the  cases  in  which  they  have  an  inde- 
pendent existence  or  in  those  in  which  each  is  part  of  a 
colony.  In  any  case  the  process  involves  the  division  of 
the  protoplast  into  two  or  many  parts,  each  of  which 
strictly  resembles  in  all  respects  its  progenitor.  The  cases 
in  which  two  new  protoplasts  result  from  the  fission  are 
the  most  numerous,  and  they  are  classed  together  gene- 
rally under  the  term  cell-division.  Of  this  there  are  various 
degrees  of  simplicity  ;  the  most  primitive  is  illustrated  by 
the  behaviour  of  some  of  the  lower  fungi,  such  as  the  Sac- 
charomycetes  or  yeasts.  Each  cell,  which  is  rounded  in 
form,  puts  out  a  lateral  protuberance  of  small  size,  which 
grows  until  it  is  of  nearly  the  same  dimensions  as  the  one 
from  which  it  sprang,  and  is  gradually  cut  off  by  the  con- 
striction of  the  cell- walls  at  the  point  of  out -growth.  Part  of 
the  nucleus  passes  into  the  new  cell,  which  becomes  thus 
separated  from  its  parent,  resembling  it,  however,  in  all 


422  VEGETABLE  PHYSIOLOGY 

respects.  This  is  known  as  gemmation  or  budding.  It  may 
go  on  so  rapidly  that  the  new  cell  in  turn  may  put  out  a 
bud  of  its  own  before  it  is  cut  off  from  its  parent,  and  in 
that  way  chains  of  cells  may  be  produced  (fig.  162). 

A  more  general  method  of  the  division  of  the  cell  is 
of  a  highly  complicated  character,  and  is  preceded  by 
an  elaborate  division  of  its  nucleus.  This  structure  we 
have  seen  consists  essentially  of  a  delicate  network  of 
fibrils  of  chromatin  embedded  in  a  hyaline  substance,  the 
whole  being  surrounded  by  a  more  or  less  well-defined  out- 
line derived  from  the  cell-protoplasm,  and  known  as  the 
nuclear  membrane.  Associated  with  it  in  some  cases  are 


G  ^    o 

*> 

abed 
Fio    162. — SACCHAROMYCES  CEREVISI^E,  OR  YEAST-PLANT,  AS  DEVELOPED 

DURING   THE   PROCESS   OP   FERMENTATION.       X    300. 

a,  b,  c,  d,  successive  stages  of  cell-multiplication. 


two  small  centrospheres.  The  process  of  division,  which  is 
known  as  Karyokinesis,  or  Mitosis,  begins  by  the  network 
of  fibrils  becoming  coarser  and  gradually  separating  to 
form  a  long  coiled  fibre.  The  nucleoli  disappear  and  the 
nuclear  membrane  ceases  to  be  distinguishable.  At  the 
same  time,  in  those  cases  in  which  centrospheres  have 
been  seen,  they  shift  their  position  and  come  to  lie  on  opposite 
sides  of  the  nucleus  at  some  little  distance  from  it.  The 
long  coiled  fibre  of  chromatin  breaks  up  into  a  number  of 
pieces,  often  V-shaped,  which  point  towards  the  centre  of 
the  nucleus.  The  number  of  these  varies  in  different  cases, 
but  is  constant  in  the  successive  divisions  of  an  individual. 
These  pieces  of  the  fibre  are  known  as  chromosomes.  The 
chromatin  in  them  is  broken  up  into  small  portions  which 
are  separated  from  each  other  by  smaller  films  of  unstain- 
able  substance. 


KEPKODUCTION 


428 


Threads  of  a  delicate  character  may  next  be  seen  to 
extend  from  one  centrosphere  to  the  other,  forming  a  hody 
known  as  the  nuclear  spindle.  The  positions  of  the  centre- 
spheres  are  called  the  poles  of  the  nucleus.  When  no 
centrospheres  can  be  detected,  the  threads  of  the  spindle 
nevertheless  converge  to  two  similarly  situated  poles.  Some 
of  the  spindle  fibres  stretch  uninterruptedly  from  pole  to 
pole,  while  others  become  in  some  way  attached  to,  or 
entangled  with,  the  chromosomes.  The  latter  travel 
along  these  threads,  with  which  their  points  are  in  contact, 
till  they  form  a  disc  across  the  spindle  (fig.  163,  &).  This 
stage  is  constant  in  all  cases  of  karyokinesis,  though  some 


FIG.  163. — STAGES  ix  KARYOKINETIC  DIVISION  OF  THE  NUCLEUS. 


a,  resting  nucleus;   6,  stage  of      

d,  commencement   of  formation  of 
across  the  cell. 


plate  ;   c,  separation  of  the  chromosomes 
11-wall ;    e,  extension  of  nuclear  spindle 


variations  of  the  antecedent  steps  have  been  observed,  the 
details  of  the  formation  of  the  disc  not  being  always  iden- 
tical. This  body  is  sometimes  called  the  equatorial  plate. 
At  some  time  during  this  preliminary  period  each  chromo- 
some splits  longitudinally  into  two,  though  the  fission  is 
generally  not  observable  till  the  equatorial  plate  is  recognis- 
able ;  the  halves  resulting  from  these  divisions  separate  into 
two  sets  in  such  a  way  that  half  of  each  original  chromo- 
some makes  its  way  towards  one  pole,  and  the  other  half 
towards  the  other.  The  two  sets  of  chromosomes  so  formed 
travel  back  along  the  spindle  fibres,  each  going  to  one  of 
the  two  poles  of  the  nucleus,  their  positions  as  they  go  being 
such  that  their  convex  sides  point  towards  the  pole  which 


424  VEGETABLE  PHYSIOLOGY 

they  are  approaching  (fig.  163,  c).  They  thus  collect 
into  two  places  which  are  determined  by  the  positions  of 
the  poles  of  the  nucleus,  or  of  the  centrospheres  if  the  latter 
are  present,  and  they  present  there  the  appearance  of  two 
somewhat  star-shaped  aggregations.  This  is  known  as  the 
diaster  stage.  The  chromosomes  at  each  pole  next  be- 
come united  by  their  ends,  and  constitute  two  new  nuclei, 
each  gradually  becoming  well  defined  by  the  appearance  of 
a  nuclear  membrane  ;  the  original  appearance  is  com- 
pleted by  the  development  of  nucleoli  in  each  new  nucleus. 
The  mechanism  of  the  movement  of  the  chromosomes 
towards  the  poles  is  not  fully  understood  at  present,  but  it 
is  held  by  some  observers  to  be  due  to  a  contraction  of  the 
spindle  fibres  to  which  the  chromosomes  are  attached.  In 
the  cases  in  which  a  centrosphere  is  present  at  the  pole  it 
takes  up  a  position  by  the  side  of  the  new  nucleus  and 
divides  into  two. 

This  process  of  karyokinesis  is  followed  in  various  ways 
by  the  production  of  a  cell- wall  between  the  two  nuclei, 
which  completes  the  division  of  the  protoplast.  In  the 
cases  in  which  the  latter  is  of  comparatively  small  diameter, 
the  spindle  fibres  become  increased  in  number,  and  form  a 
barrel-shaped  body  whose  short  diameter  stretches  com- 
pletely across  the  cell  (fig.  163,  d,  e)  till  the  spindle  is  in 
contact  with  the  lateral  cell-walls.  Granules  which  have 
been  floating  in  the  cell-protoplasm  are  to  be  seen  stream- 
ing along  the  spindle  fibres  till  they  form  a  plate  stretching 
across  the  cell  from  wall  to  wall.  From  this  plate  the 
septum  of  cellulose  and  its  associated  substances  is  formed. 

In  certain  cases  the  spindle  does  not  reach  completely 
across  the  cell.  It  is  then  at  first  in  contact  with  one  side 
only,  and  the  new  wall  begins  to  be  formed  there  in  the 
same  way  as  in  the  case  described.  It  then  detaches  itself 
from  the  part  of  the  new-formed  wall  which  is  in  contact 
with  the  old  membrane  and  moves  gradually  across  to  the 
opposite  side  of  the  cell,  the  new  wall  being  completed  as  it 
goes.  The  spindle  then  disappears. 


BEPKODUCTION  425 

In  some  of  the  Thattophytes  the  new  wall  is  formed  with- 
out the  intervention  of  a  spindle.  After  the  two  new  nuclei 
have  taken  up  their  positions,  the  new  wall  arises  midway 
between  them  as  a  ring-like  outgrowth  from  the  original  cell 
membrane,  and  gradually  grows  inwards  till  it  is  complete. 

In  the  divisions  of  the  protoplasts  which  constitute  a 
coenocyte  the  nuclear  divisions  are  not  followed  by  the 
construction  of  any  cell-walls,  so  that  the  limits  of  each 
protoplast  are  not  well  denned.  In  some  cases  indeed 
they  are  indistinguishable. 

The  reproduction  of  the  protoplast  is  sometimes  attended 
by  the  production  of  not  two  but  several,  which  appear 
simultaneously.  Such  a  case  is  illustrated  by  the  forma- 
tion of  the  endosperm  in  the  embryo  sac  of  the  Phanero- 
gams. It  is,  however,  only  a  modification  of  the  process 
already  described.  The  division  of  the  original  nucleus  is 
followed  by  the  disappearance  of  the  spindle  ;  the  daughter 
nuclei  divide  in  turn,  and  the  process  is  continued  until  a 
large  number  of  free  nuclei  lie  embedded  in  the  protoplasm 
of  the  cell.  These  then  become  connected  with  each  other 
by  the  simultaneous  development  of  connecting  fibrils  or 
small  spindles  like  the  first,  and  cell  plates,  which  later 
become  cell  membranes,  arise  across  them  as  in  the  case 
described.  The  protoplasts  so  formed  exhibit  no  differentia- 
tion among  themselves,  but  are  all  alike  in  appearance, 
structure,  and  fate. 

This  modification  of  the  process  of  reproduction  of  the 
protoplast  is  known  as  free  cell  formation,  and  in  many  cases 
it  is  attended  by  a  specialisation  of  function,  which  will  be 
alluded  to  a  little  later, 

In  many  cases  of  the  reproduction  of  such  plants  as  con- 
sist of  enormous  numbers  of  protoplasts  variously  arranged 
and  differentiated,  we  have  to  recognise  essentially  no  other 
process  than  the  multiplication  of  the  protoplasts  by  such 
means  as  we  have  just  described.  Generally  in  these 
cases  some  part  of  the  parent  plant  becomes  detached 
and  grows  at  once  into  the  new  individual.  We  have  seen 


426  VEGETABLE  PHYSIOLOGY 

that  this  is  the  regular  method  of  the  multiplication  of  the 
yeast-plant,  where  each  division  of  a  protoplast  brings 
into  being  a  new  individual.  The  process  can  be  noticed 
through  all  the  families  of  the  vegetable  kingdom,  though 
as  we  advance  upwards  in  the  scale  the  separated  body 
becomes  more  and  more  complex.  We  have  the  gemmae 
of  certain  Algae  and  Bryophyta,  which  are  multicellular  ; 
we  have  in  certain  Mosses  branches  which  become  detached 
by  the  dying  off  of  the  shoot  behind  them.  Many  Ferns 
develop  buds  upon  the  pinnae  of  some  of  their  leaves,  which 
when  separated  from  the  latter  grow  into  complete  ferns. 
Among  the  Phanerogams  we  notice  a  great  variety  of  this 
method  of  reproduction,  many  structures  being  developed 
normally  to  secure  it,  while  others  can  be  made  to  lead  to 
it  by  artificial  means.  We  have  the  propagation  of  plants 
normally  by  the  formation  and  separation  of  tubers,  buds, 
and  conns  ;  by  the  young  plants  which  are  developed  from 
the  nodes  of  runners  and  stolons.  The  artificial  method  of 
bringing  it  about  is  illustrated  by  cuttings,  which  are  pieces 
of  the  stem,  bearing  buds  ;  these,  when  detached  and  planted 
in  suitable  soil,  put  out  adventitious  roots  from  the  base 
of  the  cutting  and  develop  into  new  plants.  Other  in- 
stances are  afforded  by  the  buds  which  many  leaves, 
e.g.  those  of  Bryophyllum  and  certain  species  of  Begonia, 
put  out  when  wounded.  These  also  develop  adventitious 
roots,  and  young  plants  arise  which  become  independent. 

This  method,  in  which  we  never  meet  with  the  prepara- 
tion of  cells  which  are  specialised  in  the  direction  of 
reproductive  powers,  is  usually  spoken  of  as  vegetative 
reproduction  or  vegetative  propagation. 

Some  curious  cases  of  it  are  known.  In  the  embryo  sac 
of  Ccelebogyne  there  is  no  fertilisation  of  a  sexual  cell  in  the 
manner  which  will  shortly  be  described,  but  still  one  or 
more  embryos  arise.  This  is  caused  by  a  vegetative  budding 
of  certain  cells  of  the  nucellus  of  the  ovule,  which  grow  into 
the  interior  of  the  embryo  sac,  and  develop  into  embryos. 

A  feature  of  vegetative  propagation   which  may  here  be 


KEPBODUCTION 


427 


FIG.    164. — ZOOSPOEE    OF 
Ulothrix.  X   500. 


emphasised  is  that  the  new  individual  is  developed  con 
tinuously  after  its  origination.  There 
is  no  resting  period,  such  as  we  find 
in  most  cases,  to  mark  the  behaviour 
of  the  more  specialised  reproductive 
cells  to  be  discussed  below. 

Apart  from  cases  of  vegetative  pro- 
pagation of  the  individual,  we  meet 
with  two  other  methods  of  repro- 
duction, both  of  which  involve  the 

preparation  of  special  cells  set  apart  for  this  purpose.  The 
first  of  these  is  characterised  by  the  fact  that  each  cell  so 
produced  is  able  to  grow,  either  at 
once  or  after  a  short  period  of  rest? 
into  a  new  plant,  which  may  or  may 
not  be  exactly  like  the  one  from 
which  the  reproductive  cell  was 
formed.  In  plants  exhibiting  the 
simple  organisation  which  we  find 
among  the  seaweeds  and  the  fungi, 
the  parent  and  the  offspring  are  in 
most  cases  precisely  similar.  The 
difference  in  this  respect  between 
them  and  plants  higher  in  the  scale 
will  be  discussed  a  little  later.  A 
good  example  of  this  mode  of  repro- 
duction, which  was  probably  the 
primitive  form,  is  afforded  by  the 
common  filamentous  Alga  Ulothrix. 
Any  protoplast  of  the  filament  can 
divide  into  a  number  of  separate 
pieces,  each  of  ovoid  shape  with  a 
pointed  end  and  furnished  there  with 
four  cilia  (fig.  1 64) .  These  new  proto- 
plasts swim  about  for  a  time  in  the  water,  then  come  to 
rest,  and  after  a  time  grow  out  into  new  filaments.  Not 
only  the  Algae  but  the  Fungi  afford  examples  of  the 


FIG.      165.  —  Two     Gosi- 
DAJSGIA  os1  AcUya 

A,  closed ;  B,  ruptured,  and 
allowing  the  zoogonidia  a  to 
escape ;  b,  mother-cells  of 
the  latter,  after  escape  of 
the  zoogonidia  from  them. 


428 


VEGETABLE  PHYSIOLOGY 


development  of  such  cells,  conspicuous  among  them  being 
Saprolegnia  and  its  allies  (fig.  165).  These  free-swimming 
protoplasts  are  known  as  zoospores  or  zoogonidia.  Each  on 
coming  to  rest  clothes  itself  with  a  cell-wall,  and  can  develop 
into  a  plant  exactly  like  the  one  from  which  it  arose.  These 
zoogonidia  are  developed  by  the  protoplasm  of  a  single  cell 
dividing  up  into  a  variable  but  often  large  number  of  separate 
protoplasts,  the  process  being  known  as  free  cell  formation. 


i-fl 


FlO.  166.— CffiNOCYTE  OF  MuC'.T,  BEAB1NG  A 
GOXIDANGIUM,  L  THIS  IS  MOBE  HIGHLY 
MAGNIFIED  IN  THE  FIGUKE  TO  THE  BIGHT. 

m,  columella  ;    I,  gonidia. 


FIG.  167. — Ascr.  frcin  Peziza. 
a,  b,  c,  d,  c,  f,  stages  in 
development.  In  /  the 
ascospores  arc  mature.  The 
slender  cells  are  barren 
hairs,  or  paraphyses.  x 
250. 


Each  protoplast  possesses  a  nucleus  derived  from  the  original 
nucleus  of  the  cell  in  which  the  formation  takes  place,  in 
the  manner  already  alluded  to. 

In  most  cases  where  these  reproductive  cells  are  met  with 
they  have  not  so  simple  a  structure  as  those  so  far  described, 
but  each  is  furnished  with  a  cell- wall.  They  are  commonly 
called  spores  or  gonidia,  and  arise  in  different  ways  upon 
the  plant,  often,  or  indeed  generally,  being  developed  in  or 
on  special  organs,  known  as  sporangia  or  gonidangia. 

The  yeast-plant  gives  us  perhaps  the  simplest  form  of 


BEPBODUCTION 


429 


this  organ.  Any  cell  can  play  the  part  ;  its  nucleus  and  proto- 
plasm divide  into  a  number  of  pieces,  frequently  four,  each  of 
which  becomes  rounded  off  and  clothed  with  a  new  cell- 
wall.  After  a  time  the  four  new  cells  are  liberated  by  the 
breaking  down  of  the  original  cell-wall.  In  more  highly 
differentiated  plants  they  are  developed  in  special  cells  or 
chambers  named  asci  (figs.  166  and  167),  in  very  variable 
numbers,  and  are  known  as  ascogonidia  or  ascospores.  In 
other  cases  they  are  produced  by  abstriction  from  a  cellular 
outgrowth  of  the  thallus  (fig.  168),  and  in  these  again  the 
number  produced  from  a  single  cell 
may  vary  within  wide  limits.  These 
are  generally  called  stylogonidia  or 
stylospores.  There  is  an  almost  in- 
finite variety  of  these  bodies  to  be 
met  with  in  different  plants,  but  the 
variety  affects  only  the  conditions  of 
their  situation  and  does  not  indicate 
any  difference  in  their  own  structure. 
They  are  unicellular  bodies,  or  simple 
protoplasts,  each  clothed  with  a  deli- 
cate cell-wall. 

These  asexual  cells  are  usually 
spoken  of  as  gonidia  when  they  arise 
upon  a  gametophyte,  and  as  spores 
when  the  sporophyte  gives  them  origin.  FIG.  IGS.  — 

The  fact  that  they  do  not  usually 

. 

germinate  till  after  a  period  ot  rest, 
though  this  is  often  not  very  pro- 
longed, suggests  that  they  originated  in  consequence  of  the 
plant  needing  certain  cells  which  should  possess  the  power 
of  passing  through  times  of  exposure  to  unfavourable  condi- 
tions without  destruction.  Such  unfavourable  conditions 
would  be  likely  to  kill  the  more  delicate  vegetative  repro- 
ductive bodies.  This  view  is  supported  by  the  fact  that 
many  of  the  lower  plants,  particularly  Yeast,  do  not  pro- 
duce spores  when  conditions  are  suitable  for  the  life  of  the 


OP  ^otium,  PRODUCED 

BY      ABSTRICTION      FROM 

STERKIMATA. 


430  VEGETABLE  PHYSIOLOGY 

ordinary  individual,  but  can  be  made  to  do  so  by  cultivating 
them  under  adverse  conditions  of  moisture,  food  supply,  &c. 

A  somewhat  similar  structure  to  the  zoogonidia  described 
is  put  out  by  the  coenocytic  Alga  Vaucheria.  It  appears 
as  a  mass  of  protoplasm,  which  becomes  separated  from 
the  contents  of  a  filament,  and  is  set  free  by  an  opening  at 
the  apex  of  the  latter.  It  is  composed  of  several  proto- 
plasts which  are  arranged  together  as  in  the  rest  of  the 
.coenocyte,  but  their  individual  outlines  cannot  be  seen. 
The  fact  that  it  is  coenocytic  is  shown  by  the  presence  of  a 
number  of  nuclei  in  the  protoplasmic  mass.  A  pair  of 
cilia  are  given  off  opposite  to  each  nucleus,  so  that  it  swims 
very  readily  in  the  water  after  its  liberation.  It  is  some- 
times called  a  Zooccenocyte.  After  a  period  of  motility  it 
comes  to  rest,  the  cilia  are  withdrawn,  and  it  becomes 
clothed  by  a  cell- wall.  The  resting  period  lasts  for  a  variable 
time,  after  which  it  develops  into  a  new  Vaucheria  filament. 

Besides  these  asexual  reproductive  bodies  other  cells 
are  produced  by  the  great  majority  of  plants,  which  are 
incapable  of  giving  rise  to  new  individuals,  unless  two  of 

them  unite  or  fuse  with  one 
,V5  another.  On  account  of  this 

7*    sft^'ff£         (/    ]/     <f*  peculiarity  they  are  known 
tJ    KJ       ,i\|//     A    /jf^         as   sexual  cells   or   gametes. 

\  I  ill//  ^    %$      W/  «? 

In      the      lowliest      forms, 
such   as   many   filamentous 
AlgaB,  they  are  produced  by 
„  the   same   filament    as    the 

Fiu.    169.  -  PART   or-    A    FILAMENT    or  asexual  Cells  Or  gonidia.      In 

Ultfhrix     FROM     WHICH     THE     GAMETES   the     g^CIl    Alga     UlotJinX    W6 

TfrTZeV  **  gametes  find    the    first    "^Cation    of 

conjugating.  these  sexual  cells.    Besides 

the    large    zoogonidia   with 

their  four  cilia,  other  smaller  free-swimming  bodies  are 
developed  in  certain  cells  of  the  filament.  They  are  pro- 
duced in  larger  numbers  and  have  only  two  cilia  each  (fig. 
169).  After  they  are  set  free  into  the  water  they  swim 


KEPKODUCTION  481 

about  for  some  time,  and  then  they  usually  fuse  together 
in  pairs,  nucleus  joining  nucleus  and  protoplasm  uniting 
with  protoplasm.  The  new  body  so  formed  is  known  as  a 
zygospore.  After  a  period  of  rest  it  can  give  rise  to  a  new 
filament.  These  free-swimming  similar  sexual  cells  are 
frequently  called  planogametes. 

In  the  Zygnemece  and  the  Mesocarpece  the  gametes  are 
solitary  and  non-motile  and  do  not  escape  from  the  cells  in 
which  they  are  formed.  Two  filaments  take  part  in  the 
fusion  of  the  gametes  ;  these  are  found  lying  close  together 
in  the  water  ;  from  a  cell  of  each  filament  a  protrusion  grows 
out  towards  the  other  and  the  two  come  into  contact  and  join, 
the  separating  walls  breaking  down.  The  contents  of  one  cell 
pass  over  into  the  other  through  the  channel  so  formed,  or  the 
contents  of  both  the  cells  meet  in  the  middle  of  the  passage  ; 
fusion  of  the  two  takes  place,  and  the  new  body,  called  as 
before  the  zygospore,  clothes  itself  with  a  cell-wall.  It  is 
liberated  after  awhile  by  the  breaking  down  of  the  wall  of 
the  structure  which  encloses  it,  and  can  then  give  rise  to  a 
new  individual.  A  similar  process  is  characteristic  of  certain 
Fungi. 

In  all  these  cases,  though  the  cells  are  sexual  cells,  the 
differentiation  of  sex  is  so  slight  that  it  is  difficult  to  speak 
of  male  and  female  gametes.  In  the  Zygnemece,  in  which 
the  formation  of  the  zygospore  takes  place  in  the  cell  of 
the  filament,  the  gamete  which  passes  through  the  passage 
may  perhaps  be  regarded  as  male  and  the  more  passive  one 
as  female.  This  differentiation  cannot  be  distinguished 
in  the  Mesocarpece,  where  both  gametes  meet  in  the  connect- 
ing passage. 

In  Ulothrix  the  differentiation  of  sex  is  even  more  rudi- 
mentary, as  it  is  not  always  necessary  for  the  fusion  to  take 
place.  If  any  cell  escapes  fusion  it  may  develop  into  a  new 
filament  independently  of  this  process.  This  fact  suggests 
that  the  sexual  cells  have  been  derived  from  asexual  ones, 
and  'are  a  later  development,  therefore,  in  the  history  of  the 
race. 


482  VEGETABLE  PHYSIOLOGY 

The  more  complete  differentiation  of  the  gametes  into 
male  and  female  can  be  observed  among  several  of  the 
families  of  the  Algae.  In  some  species  of  Ectocarpus  and 
Cutleria  the  gametes  are  much  like  those  of  Ulothrix,  but 
some  are  smaller  than  the  others.  The  larger  ones  come 
to  rest  soonest,  and  lose  their  cilia  ;  one  of  the  smaller 
more  motile  ones  then  fuses  with  each  of  the  larger.  We 
can  in  this  case  speak  of  the  larger  as  female  and  the  smaller 
as  male.  The  differentiation  is  still  very  rudimentary, 
as  in  the  event  of  no  fusion  taking  place  the  female  cell  can 
still  develop  into  a  new  plant. 


Fro.    170.— OOGONIUM  OF  Fuctis,  CON-  FIG.    171.— AN  OOSPHEEE  OF  Fucus 

TAINING    EIGHT     OOSPHERES.        (After  SURROUNDED      BY      ANTHEROZOIDS. 

Thuret.)  (After  Thuret.) 


The  most  complete  differentiation  of  the  gametes  can 
be  traced  in  the  higher  members  of  the  Algae.  The  females 
become  larger  and  cease  to  develop  cilia,  the  males  remain 
small  and  motile.  The  former  are  then  called  oospheres 
and  the  latter  antherozoids  or  spermatozoids.  A  good 
example  of  this  stage  of  differentiation  is  afforded  by  Fucus 
(figs.  170  and  171). 

The  structures  or  organs  in  which  the  sexual  cells  of 
these  plants  are  formed  are  known  as  gametangia.  When 
the  gametes  are  distinctly  male  and  female  the  gametangia 
in  which  they  are  developed  are  termed  antheridia  and 
oogonia  respectively. 


REPRODUCTION 


433 


In  the  group  of  Fungi  similar  differentiation  of  gametes 
occurs,  but  motile  antherozoids  are  very  rare,  confined 
indeed  to  the  genus  MonoUepharis.  In  many  cases  they 
are  undifferentiated  masses  of  protoplasm  which  do 
not  escape  from  their  antheridia,  but  are  conducted 
directly  from  it  into  the  female  organ,  where  the  process 
of  fusion  takes  place.  In  Pyihium  the  oogonium  is  a 
swelling  at  the  end  of  a  hypha,  which  is  cut  off  from  the 
rest  by  a  transverse  wall.  Its  contents  divide  up  into  an 
oosphere  and  a  certain  amount  of  protoplasm,  which  sur- 
rounds the  sexual  cell.  The  antheridium  is  another  hyphal 
branch,  which  becomes  closely  pressed  to  the  oogonium. 
A  tube  is  put  out  by  the  antheridium,  which  perforates  the 
wall  of  the  oogonium,  and  the  male  cell,  which  is  formed 
in  the  same  way  as  the  female  one, 
passes  over  into  the  female  organ  and 
fuses  with  the  oosphere. 

In  some  other  Fungi  a  similar 
arrangement  of  the  organs  is  brought 
about,  but  the  male  cell  does  not  pass 
over  into  the  oogonium. 

A  curious  variation  is  seen  in  the 
red  seaweeds,  the  Rhodophycece.  The 
female  organ,  known  as  a  procarpium, 
does  not  produce  any  differentiated 
oosphere,  but  the  contents  of  the 
male  cell  pass  by  means  of  an  elon- 
gated structure,  called  a  trichogyne 
(fig.  172),  into  its  interior  and  appa- 
rently fuse  with  the  whole  of  its 
protoplasm.  The  male  cell  in  these 
plants  is  not  naked  as  in  other  cases, 
but  has  a  cell-  wall.  A  somewhat  simi- 
lar condition  is  met  with  among  the  Ascomycetes,  though 
whether  fusion  of  the  contents  of  the  cells  takes  place  is 
disputed. 

Except  in  the  Angiosperms  the  gametangia  of  the  plants 

28 


172.-PKOCAKWUM  OF 

A  RED  SEAWEED. 

tr,  trichogyne. 


434 


VEGETABLE  PHYSIOLOGY 


above  the  Thallophytes  are  known  as  antheridia  and 
archegonia  respectively.  An  archegonium  is  a  more  complex 
structure  than  an  oogonium,  being  composed  of  many  cells 
and  showing  differentiation  into  a  venter  and  a  neck  (fig. 
173).  It  contains  only  a  single  oosphere. 

The  sexual  cells  differ  from  the  great  majority  of  asexual 
ones  in  never  possessing  cell-walls.  The  only  cases  in 
which  they  are  clothed  with  them  are  those  of  the  Rhodo- 
phycece  and  the  Ascomycetcs  already  alluded  to.  In  both 
these  groups  the  male  gametes  are  the  only  ones  that 


FIG.  173. — DEVELOPMENT  OF  THE  ARCHEGONIUM  OF  THE  FERN. 

1,  2,  4,  5,  7,  8,  9,  Successive  stages.     3,  6  transverse  sections  of  the 
neck  region  of  4  and  5. 


have  them ;  the  females,  as  we  have  seen,  not  being 
differentiated. 

The  fusion  of  the  gametes  is  known  as  conjugation  when 
they  are  alike,  and  as  fertilisation  when  they  are  distinctly 
male  and  female.  The  resulting  body  is  termed  a  zygote ; 
it  is  a  zygospore  when  it  is  produced  by  conjugation,  and  an 
oospore  when  it  is  the  result  of  fertilisation. 

In  the  more  lowly  organised  forms  it  generally  happens 
that  both  sexual  and  asexual  reproductive  cells  may  be 
produced  upon  the  same  individual.  An  exception  is 
found  in  the  Fucacece,  the  members  of  which  do  not  develop 
any  asexual  cells.  While  it  is  possible,  however,  for  many 


BEPKODUCTION  435 

plants  to  produce  both  gonidia  and  gametes,  it  is  more 
usual  for  them  to  bear  the  former  only.  So  for  a  long 
series  of  individuals  reproduction  is  brought  about  asexually 
by  gonidia.  Then  for  some  reason  an  individual  produces 
gametes,  and  the  series  is  interrupted  by  the  occurrence 
of  sexual  reproduction.  This  is  generally  followed  by  a 
further  series  like  the  first.  We  have  here  an  instance  of  a 
kind  of  alternation  of  generations,  which  is,  however, 
irregular  and  intermittent.  As  all  the  members  of  the 
series,  whether  producing  gonidia  or  gametes,  are  essentially 
similar  or  homologous,  this  is  often  spoken  of  as  homologous 
alternation  of  generations. 

The  forms  which  we  have  discussed  appear  all  to  be 
capable  of  producing  gametes  if  conditions  require  them. 
They  are  accordingly  termed  gametophytes,  and  are  dis- 
tinguished as  actual  or  potential  as  they  do  or  do  not  give 
rise  to  sexual  cells. 

In  plants  which  are  higher  in  the  scale  the  production 
of  both  sexual  and  asexual  reproductive  cells  ceases  to  be 
possible  upon  the  same  individual,  and  we  find  consequently 
that  the  plant  exhibits  two  phases  in  its  life  cycle,  one  of 
which  is  characterised  by  the  production  of  sexual  and  the 
other  of  asexual  cells.  How  this  sharply  marked  separation 
arose  is  still  a  matter  of  controversy  which  we  need  not 
here  enter  into.  The  two  forms,  however,  differ  essentially, 
one  being  capable  normally  of  producing  only  gametes, 
the  other  of  giving  rise  only  to  spores.  The  zygote  arising 
on  the  gametophyte  from  the  sexual  fusion  is  only  capable 
of  originating  a  form  which  bears  spores,  while  the  spore  can 
only  develop  a  form  on,  which  sexual  cells  arise.  The 
asexual  form  in  the  life  cycle  is  known  as  the  sporophyte. 
The  occurrence  of  gametophyte  and  sporophyte  regularly 
in  turn,  as  described,  is  known  as  antithetic  alternation  of 
generations.  It  is  of  constant  and  regular  occurrence  in  all 
the  groups  of  plants  above  the  Thallophytes. 

The  existence  of  a  sporophyte,  or  form  which  is  never 
capable  of  bearing  gametes,  is  still  a  matter  of  discussion 

28* 


436  VEGETABLE  PHYSIOLOGY 

as  far  as  the  Thallophytes  are  concerned.  There  are  indi- 
cations of  its  origination  in  that  group,  but  they  are  ex- 
tremely rudimentary,  and  occur  in  families  which  are  widely 
separated  from  each  other.  The  gametophyte  was  doubtless 
the  primitive  form  of  the  plant,  and  in  some  way  or  other 
the  sporophyte  took  its  origin  from  it.  Certain  phenomena 
which  may  represent  stages  in  the  process  can  still  be 
observed.  In  (Edogonium  the  fertilised  cell  does  not  grow 
out  into  a  new  filament,  but  produces  in  its  interior  four 
zoospores  which  escape  from  it,  and  after  a  period  of  rest 
germinate  and  produce  new  plants.  The  fertilised  cell  here 
may  perhaps  represent  the  sporophyte,  reduced,  however,  to 
a  single  sporangium.  An  even  simpler  stage  of  develop- 
ment may  perhaps  be  recognised  in  Spirogyra,  where  the 
nucleus  of  the  fertilised  cell  divides  into  four,  though  no 
definite  cells  are  formed.  On  germination  of  the  zygote, 
however,  only  one  filament  grows  out.  A  more  complex 
structure  is  formed  in  Coleochcete ;  the  zygote  becomes 
invested  with  a  covering  derived  from  the  adjacent  cells, 
and  after  sinking  to  the  bottom  of  the  water,  it  germinates, 
producing  inside  its  coating  a  small  mass  of  cells,  each  one 
of  which  liberates  a  spore  which  is  furnished  with  cilia. 
Other  complex  structures  are  found  as  the  result  of  the 
growth  and  development  set  up  by  fertilisation  in  the 
Bhodophycece.  These  are  known  as  cystocarps,  and  they  have 
been  held  to  represent  the  sporophytes  of  those  plants.  It 
is  important  to  notice,  however,  both  in  their  case  and  in 
that  of  Coleochcete,  that  only  part  of  the  structure  in  most 
cases  is  derived  from  the  contents  of  the  fertilised  cells, 
the  rest  coming  from  other  cells  of  the  tissue  of  the  game- 
tophyte. As  we  have  seen,  the  sporophyte  in  the  higher 
plants  is  entirely  derived  from  the  zygote. 

The  antithetic  alternation  of  generations  is  seen  most 
clearly  in  the  groups  of  the  Mosses  and  Ferns.  In  the 
former  the  Moss  plant  is  the  gametophyte,  the  so-called 
capsule  or  iheca  with  its  stalk  is  the  sporophyte.  In  the 
Ferns  the  sporophyte  is  the  predominant  form  and  takes 


KEPBODUCTION 


437 


on  the  chief  vegetative  functions,  while  the  gametophyte 
is  the  small  prothallium  (fig.  174). 

Even  without  going  beyond  the  Ferns  we  can  notice  as 
we  pass  through  the  several  divisions  of  the  vegetable 
kingdom  that  the  predominant  form  of  the  plant  has  changed. 
In  the  Thallophyta  it  is  always  the  gametophyte ;  the 
sporophyte  is  not  universal  there,  and  is  never  more  than 
a  small  structure,  which  nearly  always  remains  attached 
to  the  gametophyte.  In  the  Bryophyta  the  two  phases  are 
more  nearly  alike  in  degree  of  development ;  the  garnet o- 


Fio.  174. — PEOTHALLTUM  (GAMETOPHYTE)  OF  FERN. 

phyte  is  always  the  vegetative  body,  though  the  sporophyte 
often  shows  the  greater  histological  differentiation.  It 
is  always  parasitic  upon  the  gametophyte  and  never 
attains  a  higher  degree  of  morphological  value  than  a 
thallus.  In  the  Pteridophyta  the  predominance  of  the 
sporophyte  is  very  marked,  and  as  higher  and  higher  groups 
of  plants  are  reached  it  becomes  still  more  pronounced, 
the  gametophyte  continuously  retrograding  and  ultimately 
being  reduced  to  microscopic  dimensions. 

We  encounter  for  the  first  time  in  the  group  of  the  Pteri- 
dophyta, the  Ferns  and  their  allies,  a  phenomenon  which 
becomes  of  constant  occurrence  in  all  groups  above  them, 


438 


VEGETABLE  PHYSIOLOGY 


and  which  leads  to  the  production  of  the  structure  known 
as  the  seed,  the  latter  being  a  special  body  produced  by  all 
members  of  the  group  of  Spermophytes  or  flowering  plants, 
and  now  marking  them  off  clearly  from  all  below  them.  The 
phenomenon  in  question  is  known  as  heterospory.  Plants 
which  exhibit  it  bear  two  kinds  of  spore,  which  differ  from 
each  other  mainly  in  their  relative  dimensions.  Some 
are  produced  in  large  numbers  in  a  sporangium  and  have 
usually  the  structure  which  has  already  been  described. 
Others  are  much  larger  than  these,  and  are  developed  either 
singly  or  in  small  numbers,  usually  four  in  a  sporangium. 
They  are  spoken  of  as  microstores  and  megaspores  respec- 
tively. In  the  Pteridophytes  the  megaspores,  when  formed, 
differ  from  the  microspores  chiefly  in  size  ;  in  the  Spermo- 
phytes they  are  never  liberated  from  the  sporangium  and 

have  consequently  thin  and 
delicate  walls. 

The  phenomenon  of 
heterospory  involves  the 
production  of  two  gameto- 
phytes  to  one  sporophyte, 
as  each  of  the  spores  pro- 
duces its  appropriate  pro- 
thallium.  The  gameto- 
phyte  arising  from  the 
microspore  gives  rise  only 
to  male  gametes,  that  from 
the  megaspore  only  to 
female  ones.  Such  plants 
show  in  their  life  cycle, 
therefore,  three  forms,  one 

sporophyte  and  two  gametophytes,  the  latter  occurring 
synchronously. 

The  male  gametes  are  free-swimming  antherozoids  in  all 

Pteridophytes  and  are  developed  in  antheridia  of  varying 

structure.  The  females  are  oospheres,  produced  in  archegonia. 

The  gradual  appearance  or  development  of  the  seed  can 


FIG.  175.— GERMINATION  OF  A  MASS  CF 
MICROSPORES  OF  Salvinia.  (After 
Sachs.) 

1,  The  mass  protruding  tubular  prothalli 
from  different  spores ;  2,  a  prothallus 
more  highly  magnified,  showing  an 
antheridium,  a ;  3,  antherozoids  in 
mother-cells  ;  4,  ruptured  antheridium. 


KEPBODUCTION 


489 


be  illustrated  by  studying  a  series  of  forms.  The  earliest 
indication  of  it,  which  we  can  find,  is  exhibited  by  the 
Hydropteridece,  of  which  Salvinia  is  a  characteristic  type. 
Salvinia  is  a  heterosporous  form,  each  microspore  of  which 
gives  rise  to  a  very  rudimentary  prothallium  bearing  only 
one  antheridium  with  four  antherozoids  (fig.  175).  The 
megaspore,  like  the  microspore,  is  liberated  from  the 
sporangium,  and  on  germination  it  produces  a  prothallium, 
part  of  which  remains  in  the  spore  and  part  protrudes  from 
it  (fig.  176).  The  inclusion  of  the  part  of  the  gameto- 
phyte  within  the  spore  was 
probably  the  first  step  to- 
wards the  formation  of  the 
seed.  The  young  sporophyte 
arises  upon  the  exposed  por- 
tion of  this  prothallium,  orig- 
inating as  in  other  cases  from 
the  zygote  produced  in  the 
archegonium  after  fertili- 
sation. 

A  more  advanced  stage  is 
seen  in  Selaginella,  which  also 
is  a  member  of  the  Pterido- 
phyta,  though  not  a  fern.  The 
heterospory  is  just  as  pro- 
nounced as  in  Salvinia.  When 
the  megaspore  is  set  free  from 
the  sporangium  and  its  germi- 
nation can  be  observed,  it  is 
found  that  more  of  the  game 
tophyte  remains  inside  the 
spore  (fig.  177).  The  process  of 
germination  begins  while  the  spore  is  still  in  the  sporangium 
and  by  the  time  the  spore  opens  the  prothallium  has  reached 
a  fair  degree  of  development. 

A  still  further  advance  is  shown  by  Isoetes,  in  which 
the  prothallium  is  developed  inside  the  spore,  which  only 


FEG.    176.— GERMINATION  OF    MEGA 
SPORE  OF  Salvinia. 

pro,  prothalUum ;  a,  young  sporo- 
phyte. The  thick  wall  of  the  spore 
has  been  ruptured  and  part  of  the 
prothallium  is  protruding. 


440 


VEGETABLE  PHYSIOLOGY 


opens  a  little  at  the  apex  when  the  archegonia  are  mature, 
in  order  that  fertilisation  may  be  possible. 

When  we  pass  to  the  Spermophytes  two  further  advances 
may  be  seen.  The  spore  never  escapes  from  the  sporangium, 
and  the  prothallium  does  not  emerge  even  in  part  from  the 
spore,  which  does  not  open.  In  these  plants  the  megaspore 
is  represented  by  the  cell  known  formerly  as  the  embryo-sac, 
the  sporangium  being  the  ovule.  Among  the  Spermophytes 
we  have  two  types  of  prothallium  which  are  characteristic 


FIG.  177. — GERMINATION  OF  MEGASPORE  OF  Sdagindla. 
arch,  archegonia ;    oos,  oospheres  ;    em',  embryo.     The  spore  has 
been  ruptured  and  the  upper  portion  removed. 


of  the  Gymnosperms  and  the  Angiosperms  respectively. 
Fig.  178  shows  the  structure  in  the  former  ;  the  spore  or 
embryo-sac  is  filled  with  the  prothallium,  formerly  called 
the  endosperm,  at  the  apex  of  which  are  several  archegonia 
each  containing  a  female  gamete  or  oosphere.  After  fertili- 
sation the  resulting  zygote  gives  rise  to  a  young  sporophyte 
or  embryo,  which  becomes  embedded  in  the  endosperm. 
The  structure  thus  formed  consisting  of  the  sporangium  or 
ovule,  with  the  solitary  spore  it  contains,  the  latter  having 
in  its  interior  the  embryo  surrounded  by  the  prothallus, 
constitutes  the  structure  known  as  the  seed.  It  becomes 


REPRODUCTION  441 

detached  from  the  parent  sporophyte  and  disseminated  in 
various  ways. 

In  the  Angiosperms  the  formation  of  the  seed  is  in  the 
main  similar  to  the  process  described,  hut  it  has  certain 
peculiar  features.  The  embryo-sac  or  megaspore  has  the 
same  structure  as  in  the  Gymnosperms  and  remains  en- 
closed in  the  sporangium  or  ovule.  The  development  of 
the  prothallium  is  different.  The  megaspore  has  a  single 


P.I. 


FIG.    178. — OVULE    OF    Pinus, 

SHOWING   THE    PROTHALLIUM; 

end,  is  THE  MEGASPORE  mac; 
arch,  archegonia. 


FIG.  179. — OVULE  OF  AN  ANGIO- 
SPERM,  SHOWING  THE  MEGASPORE  ; 
mac,  WITH  ITS  PROTHALLIUM; 

005,  OOSPHERE. 


nucleus  as  in  other  cases.  When  germination  begins  this 
divides  into  two,  one  of  which  travels  to  each  end  of  the 
ovoid  spore.  Each  of  these  gives  rise  by  two  successive 
divisions  to  a  group  of  four  nuclei,  and  a  single  nucleus  from 
each  group  returns  to  the  centre  of  the  cell,  where  the  two 
fuse  together.  These  are  often  termed  the  polar  nuclei. 
At  this  stage  the  prothallium  ceases  to  undergo  any  change 
(fig.  179)  ;  it  consists  of  a  group  of  three  nuclei  at  the  apex, 
known  as  the  egg  apparatus  ;  another  group  at  the  base, 


442  VEGETABLE  PHYSIOLOGY 

termed  the  antipodal  cells  ;  and  the  nucleus  in  the  centre 
which  is  the  result  of  the  fusion  of  the  polar  nuclei,  and  is 
called  the  definitive  nucleus  of  the  embryo-sac.  Each  nucleus 
is  surrounded  by  protoplasm,  the  egg  apparatus  in  parti- 
cular showing  three  well-defined  naked  or  primordial  cells. 
The  antipodal  cells  become  clothed  with  cell- walls.  There 
is  a  certain  amount  of  protoplasm  existing  in  the  spore, 
lying  around  the  wall  and  forming  bridles  across  it,  con- 
necting the  peripheral  substance  with  that  in  the  centre  in 
which  the  definitive  nucleus  is  resting. 

There  are  no  apparent  archegonia  :  the  oosphere  is  one 
of  the  three  cells  of  the  egg  apparatus,  the  other  two  being 
known  as  the  synergidce.  The  oosphere  is>  a  product  of 
the  last  division  of  the  original  upper  nucleus,  the  other 
half  being  the  polar  nucleus  which  takes  part  in  the  fusion 
described. 

As  in  the  Spermophytes  the  spore  always  remains  enclosed 
in  the  ovule  or  sporangium,  and  its  prothallium  with  the 
female  organs  is  enclosed  in  it,  the  method  of  fertilisation 
of  the  oosphere  by  a  free-swimming  antherozoid  is  impractic- 
able. The  problem  of  bringing  the  sexual  cells  together  is 
met  by  causing  the  germination  of  the  microspore  to  take 
place  on  some  part  of  the  tissue  near  the  megaspore,  and  in 
almost  all  cases  by  its  prothallium  taking  the  form  of  a  tube, 
which  grows  down  through  the  tissue  of  the  parts  sur- 
rounding the  megaspore  to  the  megaspore  itself.  This 
tubular  prothallium,  known  as  the  pollen  tube,  bears  usually 
two  male  gametes,  which  are  thus  brought  into  the  neigh- 
bourhood of  the  archegonia  or  the  egg  apparatus  respectively. 
In  a  few  species  of  the  Gymnosperms  the  male  gametes  are 
ciliated  antherozoids,  but  usually  they  are  two  conspicuous 
nuclear  masses  associated  with  a  little  cytoplasm. 

In  the  Gymnosperms  fertilisation  is  brought  about  by 
the  entry  of  a  male  gamete  into  an  archegonium.  In  the 
species  with  ciliated  gametes  these  are  not  transferred 
through  pollen  tubes.  The  pollen  grain  or  microspore 
penetrates  into  the  microphyll  of  the  ovule  and  is  drawn 


REPRODUCTION  443 

into  a  special  chamber  inside  the  integument.  Here  it 
puts  out  tubes,  but  the  tissue  of  the  ovule  breaks  down, 
drawing  the  pollen  grain  close  to  the  necks  of  the  arche- 
gonia.  The  cavity  becomes  filled  with  liquid,  and  the  ciliated 
gametes,  when  discharged  from  the  germinating  pollen-grain, 
reach  the  female  cells  by  swimming.  The  pollen  chamber 
receives  the  pollen  in  all  the  Gymnosperms,  but  in  those 
with  nonciliated  gametes  the  pollen  tube  acts  as  the  carrier 
of  the  gametes  to  the  archegonia. 

In  the  Angiosperms  one  of  the  generative  nuclei  fuses 
with  the  oosphere.  In  many  families  the  other  one  has  been 
seen  to  fuse  with  the  definitive  nucleus. 

After  the  fertilisation  of  the  oosphere  in  both  cases  an 
embryo  is  developed  from  it,  which  remains  enclosed  in  the 
spore.  In  the  Angiosperms  fertilisation  is  followed  not 
only  by  the  formation  of  an  embryo,  but  also  by  a  large 
development  of  tissue  arising  in  consequence  of  repeated 
divisions  of  the  definitive  nucleus,  so  that  the  spore  con- 
tains a  massive  so-called  endosperm  in  addition  to  the 
embryo,  the  latter  being  usually  embedded  in  the  former. 
This  so-called  endosperm  has  thus  a  different  morpho- 
logical value  from  the  endosperm  of  the  gymnospermous 
plant. 

One  of  the  most  remarkable  features  about  the  struc- 
ture and  behaviour  of  the  seed  is  the  fact  that  soon  after 
the  embryo  is  formed  it  enters  upon  a  period  of  rest,  which 
in  some  cases  is  very  prolonged.  During  this  period  the 
seed  becomes  detached  from  the  parent  plant.  The  re- 
sumption of  its  growth  and  development  is  known  as  the 
germination  of  the  seed.  This  resting  period  does  not 
occur  during  the  development  of  the  sporophyte  in  the 
Cryptogams. 

The  embryo  frequently  attains  a  considerable  size  before 
its  resting  period  commences.  In  this  case  it  absorbs 
the  contents  of  the  cells  of  a  considerable  part,  or  some- 
times the  whole,  of  the  endosperm,  so  that  it  fills  more  or 
less  completely  the  cavity  of  the  spore. 


444  VEGETABLE  PHYSIOLOGY 

The  seed  may  thus  be  a  very  complex  structure  ;  it  may 
consist  of  the  following  parts  : 

(1)  The  testa  or  skin,  derived  from  the  integuments  of 

the  ovule. 

(2)  The  perisperm,  or  remains  of  the  "body  of  the  megas- 

sporangium. 

(3)  The  embryo-sac  or  megaspore. 

(4)  The  endosperm  derived  from  the  definitive  nucleus. 

(5)  The  embryo  developed  from  the  zygote. 

The  antipodal  cells  generally  disappear  during  the  develop- 
ment. (2)  and  (4)  may  be  absent,  having  been  absorbed  by 
the  megaspore  or  by  the  embryo  respectively  during  their 
development.  If  either  or  both  are  present  the  seed  is 
said  to  be  albuminous,  the  term  albumen  embracing  both 
perisperm  and  endosperm. 

In  the  seeds  of  the  Gymnosperms  the  endosperm  repre- 
sents the  prothallium  or  gametophyte. 

The  formation  of  the  seed  we  have  seen  to  depend 
upon  the  fusion  of  the  sexual  cells  or  gametes.  This  process 
is  a  very  widespread  one,  and  in  all  plants  which  exhibit  an 
autothetic  alternative  of  generations  is  the  starting-point 
of  the  development  of  the  young  sporophyte.  The  mode 
of  bringing  the  gametes  together  varies  with  the  habit  of  life 
of  the  plant.  Where  the  male  gamete  is  a  motile  antherozoid 
it  makes  its  way  to  the  oosphere  by  means  of  its  cilia,  which 
enable  it  to  swim  freely  in  water.  In  those  forms  with  a 
terrestrial  habit,  such  as  the  Bryophytes  and  Pteridophytes, 
in  which  the  antherozoid  is  ciliated  (fig.  180),  fertilisation 
can  only  be  brought  about  when  the  gametophytes  are 
moistened,  as  is  the  case  from  time  to  time.  The  anthero- 
zoids  sometimes  arise  in  antheridia  upon  the  same  gameto- 
phyte as  the  archegonia  with  their  oospheres,  sometimes 
upon  different  ones.  In  the  heterosporous  forms  of  course 
the  latter  is  always  the  case.  A  large  number  of  such 
gametophytes,  bearing  male  and  female  cells  respectively, 
are  always  produced  in  the  immediate  neighbourhood  of 
one  another,  so  that  the  transport  of  the  antherozoids  to 


KEPKODUCTION 


445 


other  prothallia  than  their  own  is  not  at  all  difficult.  After 
their  liberation  they  are  attracted  to  the  archegonia  by 
some  constituent  of  the  mucilaginous  matter  which  is 
excreted  from  their  necks  when  they  open  (fig.  181).  In 
the  Mosses  this  has  been  ascertained  to  be  cane-sugar,  in 


FIG.  180.— ANTHEROZOIDS  OP  Moss  (A)  AND  FERN  (B). 

the  Ferns  it  is  malic  acid  or  one  of  its  salts.  In  the  Bhodo- 
phycese  and  such  Ascomycetes  as  exhibit  sexual  reproduc- 
tion, the  passive  male  gamete,  often  called  a  spermatium 
instead  of  an  antherozoid,  is  floated  to  the  female  organ  or 
its  trichogyne  by  currents  in  the  water. 


FIG.  181. — DEVELOPMENT  OF  THE  ANTHERIDIUM  IN  THE  FERN.    (After  Kny.) 

1,  2,  4,  5,  7,  8,  9,  Successive  stages.     3,  6  transverse  sections  of  the 
neck  region  of  4  and  5. 

In  the  Spermophytes,  where  the  female  gametophyte  is 
always  attached  to  the  parent  sporophyte,  such  a"  means 
of  fertilisation  is  of  course  impossible.  For  fertilisation  to 
take  place  it  is  necessary  that  the  two  gametophytes  shall 
be  produced  in  close  propinquity  to  each  other.  This  is 
effected  by  the  bringing  together  of  the  two  spores  con- 
cerned in  developing  them.  The  microspore  or  pollen-grain 
is  carried  by  various  means  to  the  neighbourhood  of  the 


446 


VEGETABLE  PHYSIOLOGY 


megasporangium  ;  in  the  Gymnosperms  it  falls  upon  the 
megasporangium  itself ;  in  the  Angiosperms  upon  the 
stigma  of  the  pistil  in  which  the  megasporangia  are  hidden. 
When  it  germinates  the  prothallium  or  gametophyte  takes 
the  form  of  a  long  tube,  which  bores  its  way  through  the 
intervening  tissues  till  it  reaches  the  megaspore  itself, 
close  to  the  archegonium  in  the  first  case,  and  to  the  oosphere 
in  the  Angiosperms,  where  there  is  no  archegonium.  In 
the  Gymnosperms  the  tube,  the  so-called  pollen-tube,  con- 
tains a  single  antheridium,  which  produces  two  gametes. 


FIG.  182. — DEVELOPMENT  OF  THE  ARCHEGONITJM  OF  THE  FERN.    (After  Kny.) 

1,  2,  4,  5,  7,  8,  9  Successive  stages ;    3,  6  transverse  sections  of  the 
neck  region  of  4  and  5. 


These  are  generally  undifferentiated  portions  of  protoplasm, 
but  in  Ginkgo,  Zamia,  and  in  some  species  of  Cycas  they  have 
been  found  to  be  ciliated  antherozoids.  In  the  Angiosperms 
there  is  no  antheridium,  but  two  gametes  which  show  no 
differentiation  are  produced  in  the  pollen-tube.  From  the 
great  preponderance  of  the  nuclear  matter  they  contain 
they  are  often  spoken  of  as  the  generative  nuclei. 

Fusion  of  the  latter,  or  of  the  antherozoi'd,  with  the 
oosphere,  becomes  possible  by  a  deliquescence  of  the  sepa- 
rating walls,  and  in  all  cases  a  single  male  gamete  fuses 
with  an  oosphere.  Where  several  oospheres  are  found 
upon  the  same  prothallium,  as  in  the  Gymnosperms,  more 


KEPKODUCTION  447 

than  one  may  be  fertilised  by  gametes  from  the  same 
pollen-tube.  This  occurs  in  certain  of  the  Cupressineae  ;  it 
is  rendered  possible  by  a  multiplication  of  the  male  gametes, 
which  takes  place  by  ordinary  processes  of  division  exhibited 
by  them  as  they  pass  down  the  tube.  Several  embryos 
may  thus  arise  in  the  seed.  Usually,  however,  only  one 
of  these  undergoes  a  normal  development. 

In  many  families  of  the  Angiosperms  the  second  of  the 
generative  nuclei  has  been  observed  to  fuse  with  the  two 
polar  nuclei  or  the  definitive  nucleus  of  the  embryo-sac. 
The  extent  to  which  this  takes  place  has  not  yet  been 
determined  and  its  interpretation  is  not  at  present  easy. 
Some  observers  hold  that  the  fusion  of  the  cells  has  nothing 
sexual  about  it,  but  is  nutritive  only  ;  others  look  upon 
the  so-called  endosperm  which  results  as  an  abortive  second 
embryo. 


448       VEGETABLE  PHYSIOLOGY 


CHAPTEE  XXVI 

REPRODUCTION    (CONTINUED) 

We  have  seen  that  the  phenomena  of  fertilisation  are 
preceded  in  the  Spermophytes  by  an  arrangement  through 
which  the  two  gametophytes,  which  give  rise  respectively 
to  the  male  and  female  sexual  cells,  are  developed  in  such 
close  proximity  that  they  ultimately  come  into  contact. 
That  which  is  produced  as  the  result  of  the  germination 
of  the  microspore,  or  pollen-grain,  is  a  tube  of  varying  length, 
which  bores  its  way  through  the  tissue  of  certain  parts  of 
the  sporophyte,  being  guided  in  some  manner  not  yet  fully 
understood,  until  it  reaches  some  part,  usually  the  apex, 
of  the  megaspore  or  embryo-sac,  in  which  synchronously 
the  prothallium  which  bears  the  oosphere  has  been  developed. 
In  the  process  of  sexual  reproduction  in  these  plants  we 
have  two  phenomena  presented,  which  have  often  been 
treated  of  as  if  they  were  inseparably  connected.  The 
first  of  these,  which  is  known  as  pollination,  involves  merely 
the  transport  of  the  pollen-grain  to  an  appropriate  position 
on  some  part  of  the  megasporophyll  or  of  the  megaspo- 
rangium  itself.  The  second,  which  may  or  may  not  follow 
the  former  one,  is  the  actual  fusion  of  the  gametes  which  are 
produced  upon  the  gametophytes  to  which  the  spores  give 
rise,  and  which  therefore  must  be  considerably  later  in  the 
time  of  its  occurrence.  This  is  what  we  have  already 
described  as  fertilisation.  • 

It   is   necessary   to   insist   on   the   distinction   between 
these  two  processes,  as  the  phrase  'the  fertilisation  of  the 


REPRODUCTION  449 

flower  '  is  frequently  somewhat  loosely  and  erroneously 
made  use  of  when  pollination  is  meant. 

We  have  seen  that  cross-fertilisation  is  as  a  rule  more 
advantageous  to  a  plant  than  the  fusion  of  gametes  which 
are  both  produced  by  the  same  individual.  In  the  same 
way  certain  advantages  are  secured  by  the  process  of  cross- 
pollination  or  the  application  of  the  pollen  of  one  flower  to 
the  stigma  of  a  different  one  of  the  same  species.  In  the 
case  of  flowering  plants  or  any  others  which  are  hetero- 
sporous,  self-fertilisation  in  the  strict  sense  is  of  course 
impossible,  as  the  male  and  female  cells  which  fuse  together 
are  necessarily  borne  upon  gametophytes  which  originate 
from  different  spores  and  cannot  thus  be  derived  imme- 
diately from  the  same  individual.  Self-pollination,  or  the 
transference  of  pollen  from  the  stamens  to  the  stigma  of 
the  same  flower,  is,  however,  possible,  and  in  many  cases 
occurs  in  the  ordinary  course  of  events.  Cross-pollination, 
or  the  bringing  together  of  spores  from  different  flowers 
of  the  same  species,  has  been  found  to  yield  more  and  better 
seeds  than  self-pollination. 

Very  many  mechanisms  have  been  developed  in  different 
plants  to  secure  this  end,  which  are  seen  to  the  greatest 
advantage  in  the  highly  developed  flowers  of  the  Angio- 
sperms.  Pollen  may  be  carried  from  flower  to  flower  by 
wind  or  water,  or  by  the  agency  of  insects  or  other  animals. 
From  this  point  of  view  flowers  have  been  classed  as 
anemophilous  or  wind-pollinated,  hydrophilous  or  water- 
pollinated,  eniomophilous  or  insect-pollinated,  and  200- 
philous  or  pollinated  by  other  animals. 

Of  these  methods  of  cross-pollination,  the  anemophilous 
and  the  entomophilous  are  most  widespread.  The  former 
is  the  more  primitive  ;  indeed,  the  latter  has  been  gradually 
supplanting  it.  We  find  cases  now  of  nearly  allied  genera 
which  illustrate  the  transition  from  the  one  to  the  other. 
Among  the  Kanunculaceae  the  flowers  of  the  genus  Thalic- 
trum  are  pollinated  by  the  wind,  while  those  of  the  more 
specialised  genera,  Aconitum  and  Delphinium,  depend  upon 

29 


450'  VEGETABLE  PHYSIOLOGY 

insects.     The  Plantains  also  afford  instances  of  the  replace- 
ment of  the  one  method  by  the  other. 

Aneraophilous  flowers  exhibit  certain  structural  features 
which  are  associated  with  their  mode  of  transference  of 
the  pollen.  It  is  produced  in  such  flowers  in  great  abun- 
dance, is  extremely  light  and  dry,  and  in  some  cases  is 
furnished  with  bladders  to  aid  its  transport.  The  receptive 
organ  is  in  some  cases  a  bulky  cone,  the  leaves  of  which 
are  separated  from  each  other,  and  from  the  common  axis, 
by  spaces  into  which  the  pollen  may  drop.  In  others  it  is 
a  much-divided  or  plumose  stigma,  often  furnished  with 
hairs,  so  that  pollen  falling  on  it  may  be  readily  retained. 
The  method,  however,  is  a  wasteful  one  and  involves  the 
production  of  a  superabundance  of  pollen.  On  the  other 
hand  anemophilous  flowers  are  always  inconspicuous  and 
of  a  comparatively  humble  type. 

Flowers  which  are  pollinated  by  insects  are  usually 
much  larger  and  more  showy  ;  they  not  infrequently  possess 
irregular  corollas,  and  are  often  very  highly  coloured  and 
provided  with  characteristic  odours.  Their  perianths,  and 
sometimes  their  sporophylls,  are  highly  modified  to  adapt 
them  to  the  habits  of  their  insect  visitors.  As  a  further 
attraction  to  the  latter  they  usually  produce  honey  in  some 
part  of  the  flower,  the  nectary,  in  such  a  situation  as  will  lead 
to  the  removal  of  pollen  by  the  insect  in  its  search  for  the 
attractive  liquid.  The  markings  on  the  coloured  perianth 
leaves  are  often  arranged  in  such  a  way  as  to  direct  the  insect 
towards  the  spot  where  the  honey  is  concealed.  The  pollen 
itself  also  is  often  the  object  of  the  insect's  visit.  Many 
special  mechanisms  to  secure  the  removal  of  the  pollen  from 
the  microsporophylland  its  deposition  on  the  stigma  of  another 
flower  are  to  be  met  with  ;  indeed,  almost  every  Natural 
Order  shows  some  modification  of  the  structure  of  the  flower 
in  this  direction.  The  consideration  of  them  in  detail, 
however,  is  beyond  our  present  purpose. 

Something  akin  to  cross-pollination  occurs  in  one  of 
the  Hydropterideae,  a  family  of  Ferns  with  an  aquatic  habit. 


BEPBODUCTION  -  451 

The  plant  in  question,  which  is  known  as  Azolla,  is  a  small 
floating  organism,  consisting  of  a  horizontal  rhizome,  some- 
times copiously  branched,  on  which  are  borne  numerous 
very  small  leaves,  which  are  partially  submerged.  It  bears 
spores  of  two  kinds,  both  kinds  produced  in  sporangia, 
which  occur  in  definite  groups  or  sori.  There  are  numerous 
microspores  in  each  microsporangium,  which,  when  mature, 
are  agglutinated  together  in  masses.  The  contents  of  a 
sporangium  usually  exhibit  two  to  eight  of  such  masses, 
each  of  them  being  known  as  a  massula.  These  are  set 
free  separately.  A  delicate  skin  surrounds  each  massula, 
and  in  some  species  this  is  furnished  with  a  number  of 
hairs  bearing  at  their  free  ends  barbed  processes  known  as 
glochidia.  The  megasporangium,  which  is  solitary  in  its 
sorus,  bears  only  a  single  megaspore.  It  is  liberated  from 
the  sporangium,  and  is  then  found  to  be  furnished  on  its 
lower  surface  with  large  spongy  bodies  which  are  developed 
from  its  outer  coat,  and  which  serve  as  floats,  enabling  it 
to  drift  about  in  the  water.  The  apex  of  the  spore  bears 
a  number  of  delicate  filaments  extending  between  the  floats. 
Both  spores  germinate  after  liberation,  each  producing  its 
appropriate  gametophyte.  The  glochidia  of  a  massula  of 
microspores  generally  catch  in  the  filaments  of  a  megaspore, 
which  may  have  arisen  on  a  different  plant,  and  the  massula 
thus  becomes  anchored  to  the  megaspore.  The  gameto- 
phytes  are  thus  brought  together,  so  that  the  gametes  can 
come  into  close  propinquity  to  each  other. 

The  mechanical  adaptations  which  have  been  described 
are,  however,  not  the  only  means  we  find  to  secure  cross- 
pollination.  There  are  peculiarities  connected  with  what 
we  may  call  the  receptivity  of  the  pistil  for  any  particular 
pollen.  Of  these  the  most  generally  occurring  is  dichogamy, 
or  the  maturing  of  the  microsporophylls  and  the  megasporo- 
phylls  of  a  flower  at  different  times.  Two  varieties  of  the 
condition  are  met  with  :  in  the  first,  known  as  protandry, 
the  stamens  with  their  pollen  are  mature,  while  the  stigma 
is  not  sufficiently  developed  to  be  pollinated.  Examples 

29* 


452  VEGETABLE  PHYSIOLOGY 

may  be  found  in  the  Gentianacese,  Onagraceae,  Campanu- 
laceae,  Composite,  &c.  In  Parnassia  the  receptive  surface 
of  the  stigma  is  not  even  formed  until  the  anthers  have 
discharged  their  pollen.  The  second  condition  is  known 
as  protogyny,  and  is  the  converse  of  the  first,  the  stigma 
withering  before  the  pollen  is  mature.  This  condition  occurs 
in  both  anemophilous  and  entomophilous  flowers  ;  certain  of 
the  Plantains  (Plantago)  and  some  grasses  (Anthoxanthum, 
&c.)  show  it  in  the  former  group,  as  does  Scrophularia 
among  the  latter. 

Something  corresponding  to  dichogamy  is  found  among 
the"  Ferns,  where  the  antheridia  and  archegonia  on  a  pro- 
thallium  do  not  mature  simultaneously.  Cross-fertilisation 
must  consequently  be  the  only  form  possible.  The  same 
peculiarity  may  be  observed  among  the  Mosses. 

Another  means  observed  in  many  cases  to  secure  cross- 
pollination  is  diclinism,  or  the  production  of  the  stamens  and 
carpels  in  different  flowers.  Diclinous  plants  may  be 
monoecious,  where  the  staminate  and  pistillate  flowers  are  on 
the  same  plant ;  dioecious,  where  they  are  on  different  plants  ; 
or  polygamous,  where  a  plant  bears  flowers  with  stamens  and 
carpels,  as  well  as  others  which  contain  only  one  or  the 
other  kind  of  sporophyll. 

The  terms  '  monoecious '  and  '  dioecious  '  are  sometimes 
applied  to  the  Cryptogams,  when  their  sexual  organs  are 
upon  the  same  or  upon  different  plants.  They  then  refer, 
of  course,  to  the  gametophytic  and  not  to  the  sporophytic 
phase  of  the  life  cycle  as  in  the  cases  just  quoted. 

Some  flowers  exhibit  a  peculiarity  of  form,  which  is 
an  adaptation  favouring  cross-pollination.  The  plants 
possess  flowers  of  two  kinds,  which  are  specially  related 
to  each  other.  The  most  familiar  instance  in  oar  own 
flora  is  the  common  Primrose,  which  has  five  stamens  and 
a  club-shaped  stigma.  In  some  flowers  the  stigma  is 
placed  just  in  the  throat  of  the  corolla,  and  the  stamens 
some  little  way  down  its  tube.  In  the  rest  of  the  flowers 
the  positions  are  reversed.  We  have  here  an  adaptation 


KEPKODUCTION  453 

to  the  visiting  insect,  for  when  it  touches  the  stamens  of  a 
short-styled  form,  it  covers  with  pollen  the  part  of  its 
body  which  will  come  into  contact  with  the  stigma  of  the 
next  long-styled  flower  it  alights  upon.  Another  portion  of 
its  body  will  be  dusted  with  the  pollen  from  the  latter,  which 
will  be  suitably  placed  to  be  deposited  upon  the  stigma  of 
the  next  short-styled  form  it  may  visit.  The  best  seeds  are 
produced  when  each  stigma  is  supplied  with  pollen  from 
stamens  occupying  a  corresponding  position  to  itself.  This 
method  of  cross-pollination  is  not  thoroughly  effective,  as 
the  insect  after  a  short  time  will  be  carrying  pollen  from 
stamens  of  both  lengths,  having  visited  several  flowers  of 
both  kinds.  The  size  of  the  pollen-grains  in  each  case  is, 
however,  correlated  with  the  features  of  the  corresponding 
stigmatic  surface,  which  helps  to  secure  the  most  advan- 
tageous result. 

This  arrangement  is  termed  heterostylism  or  dimorphism, 
of  which,  however,  it  is  only  one  form.  Lyihrum  Salicaria 
is  tnmorphic,  bearing  two  sets  of  stamens  of  different 
lengths,  and  a  style  which  differs  from  both.  There  are 
three  modes  of  arrangement  of  these  organs,  and,  as  in  the 
Primrose,  the  most  serviceable  pollination  is  that  which 
takes  place  when  pollen  from  a  stamen  of  a  particular 
length  is  applied  to  a  stigma  in  a  corresponding  position. 

Other  arrangements  are  physiological  rather  than  struc- 
tural. Of  these  the  strangest  is  what  is  called  prepotency. 
When  a  stigma  of  a  flower  exhibiting  this  property  is 
pollinated  by  pollen  from  its  own  stamens,  and  at  the  same 
time  by  pollen  taken  from  another  flower,  the  latter  is 
always  the  originator  of  the  gamete  by  which  fertilisation 
is  effected.  Some  flowers  show  self -sterility — that  is,  their 
oospheres  are  incapable  of  being  fertilised  by  generative 
nuclei  developed  from  their  own  pollen ;  in  some  few  cases 
their  own  pollen  acts  as  a  poison  to  them. 

Though  cross-pollination  is  generally  most  advantageous 
it  is  not  universal.  Self-pollination  occurs  in  many  plants  ; 
in  some,  indeed,  special  means  have  been  developed  to  secure 


454  VEGETABLE  PHYSIOLOGY 

it,  either  exclusively,  or  in  cases  in  which  cross-pollination 
fails  to  be  effected.  Only  one  of  these  need  here  be  alluded 
to  :  this  is  cleistogamy,  or  the  production  of  special  flowers 
which  do  not  open,  in  addition  to  the  normal  ones.  The 
most  conspicuous  instances  of  this  are  afforded  by  several 
species  of  the  genus  Viola.  In  one  of  these  flowers  the 
pollen-grains  often  put  out  their  pollen-tubes  while  they 
are  in  the  sporangia,  and  the  tubes  grow  towards  the  stigma, 
penetrating  it  and  reaching  the  ovules  as  in  the  case  of  the 
normal  flower,  fertilisation  resulting  in  the  same  way. 

The  process  of  pollination  is  followed  in  the  ordinary 
course  of  events  by  the  germination  of  the  microspore  or 
pollen  grain.  The  facts  that  it  grows  upon  the  substratum 
of  the  stigmatic  surface,  and  that  the  resulting  gametophyte 
or  pollen-tube  is  often  of  considerable  length,  mark  a  great 
difference  between  it  and  the  gametophytes  of  the  vascular 
cryptogams.  It  becomes,  indeed,  a  parasite  feeding  upon  a 
host  plant  during  the  greater  part  of  its  development. 

The  course  of  events  in  the  germination  of  the  pollen- 
grain  appears  to  be  the  following  :  At  the  outset  it  absorbs 
water  from  the  moist  surface  of  the  stigma  and  swells,  its 
protoplasm  becoming  generally  more  granular.  It  almost 
simultaneously  absorbs  such  food  as  the  surface  of 
the  stigma  can  supply,  usually  some  kind  of  sugar.  Most 
pollen-grains  contain  a  certain  amount  of  reserve  food, 
frequently  starch  or  sugar,  or  both.  The  process  of  absorp- 
tion is  followed  by  the  secretion  of  enzymes,  which  can  act 
upon  these  reserve  materials,  the  most  prominent  of  which 
are  diastase  and  invertase.  The  former  seems  to  be  the 
most  widespread,  but  the  latter  is  far  from  uncommon.  In 
some  cases  both  enzymes  are  developed.  The  outer  coat 
of  the  grain  then  bursts,  and  the  inner  one  begins  to  pro- 
trude, probably  in  consequence  of  the  hydrostatic  pressure 
set  up  by  the  water  that  has  been  absorbed.  Usually  only 
one  such  tube  protrudes,  though  occasionally  several  are 
developed.  Intra-cellular  digestion  of  the  reserve  materials 
follows,  and  the  tube  grows  at  their  expense.  The  increased 


EEPEODUCTION  455 

nutrition  is  followed  by  a  further  increase  of  the  enzymes, 
which  is  sometimes  preceded  by  a  temporary  diminution. 
This,  however,  does  not  last  long,  and  soon  a  considerable 
increase  can  be  observed.  In  some  pollen-tubes  such  as  those 
of  the  Lily,  in  whose  pollen  starch  granules  are  abundant, 
the  process  of  the  digestion  of  the  starch  can  be  observed 
taking  place  as  the  granules  move  along  the  tube  during 
its  elongation.  Soon  an  excretion  of  the  enzyme  into  the 
tissues  of  the  style  takes  place,  and  the  reserve  materials 
which  are  stored  in  the  style  are  gradually  digested  as  the 
tube  advances,  thus  ministering  to  its  nutrition,  absorption 
of  the  products  of  the  digestion  being  effected  by  the  tube. 
The  latter  in  most  cases  makes  its  way  to  the  micropyle  of 
the  ovule,  and  by  this  channel  reaches  the  embryo-sac  or 
megaspore.  At  this  period  the  latter  contains  its  gameto- 
phyte,  or  prothallium,  at  the  apex  of  which  the  oosphere 
or  female  gamete  occurs.  The  tip  of  the  pollen-tube  comes 
in  contact  with  the  wall  of  the  embryo-sac  close  to  the 
oosphere.  It  then  contains  two  gametes,  which  are  un- 
differentiated  masses  of  protoplasm,  each  with  a  v£ry  large 
nucleus.  The  separating  walls  become  deliquescent  and 
are  absorbed,  and  one  of  the  male  gametes  fuses  with  the 
oosphere,  forming  as  before  a  zygote,  while  the  other  often 
fuses  with  the  definitive  nucleus,  as  has  already  been 
described. 

In  a  few  cases  the  pollen-tube  makes  its  way  to  the  base 
of  the  embryo-sac  and  burrows  through  its  contents,  reach- 
ing the  oosphere  from  below.  This  has  been  observed 
particularly  in  Casuarina  and  in  certain  of  the  forest  trees. 

A  few  variations  of  this  process  have  been  observed 
among  the  Gymnosperms.  As  the  plants  of  this  group  are 
all  diclinous,  self-pollination  is  of  course  impossible.  The 
agent  of  pollination  is  usually  the  wind,  and  the  pollen- 
grain  in  these  plants  falls  upon  the  micropyle  of  the  ovule, 
there  being  no  ovary  and  consequently  no  stigma.  The 
growth  of  the  tube  is  slow,  sometimes  extending  over 
several  months.  Indeed. in  some  cases  the  sporangium  is 


456  VEGETABLE  PHYSIOLOGY 

detached  from  the  parent  plant  before  it  has  reached  the 
embryo-sac,  from  which  it  is  separated  by  a  bulky  portion  of 
the  nucellus  or  body  of  the  sporangium.  In  Girikgo,  Zamia, 
and  in  a  species  of  Cycas,  the  male  gametes  are  definite 
antherozoids,  furnished  with  cilia.  In  most  of  the  Gymno- 
sperms,  however,  this  degree  of  differentiation  has  not  been 
observed.  The  character  of  the  female  gametophyte  has 
been  already  described. 

.  Though  cross-fertilisation  is  seen  to  be  most  advantageous 
throughout  the  vegetable  kingdom,  it  is  only  possible  within 
certain  limits.  For  a  new  individual  to  be  produced,  the 
sexual  cells  taking  part  in  the  process  must  have  a  certain 
degree  of  relationship  ;  for  instance,  the  antherozoid  of  a 
moss  cannot  fertilise  the  oosphere  of  a  fern.  The  most 
favourable  degree  of  relationship  is  that  the  two  gametes 
shall  be  produced  by  different  plants  of  the  same  species. 
Such  a  union  results  in  greater  numbers  of  offspring  and 
in  the  possession  of  greater  vigour  by  them.  Plants  not  so 
closely  related  may,  however,  produce  offspring  ;  we  may 
have  the  union  of  gametes  of  plants  standing  to  each  other 
in  the  relation  of  varieties  of  the  same  species,  or  very 
frequently  of  distinct  species  belonging  to  the  same  genus, 
or  even  of  species  of  different  genera.  Such  fertilisation  is 
known  as  hybridisation. 

Hybrids,  the  offspring  of  such  fertilisation,  generally 
exhibit  peculiarities  of  form  and  structure  intermediate 
between  those  of  their  parents  ;  they  are  generally  fertile 
with  either  of  the  parent  species,  but  not  usually  so  with 
another  hybrid,  or  to  a  much  smaller  extent.  When 
crossed  with  one  of  the  parent  forms  the  offspring  tend  to 
revert  to  that  form. 

The  immediate  result  both  of  pollination  and  of  fertili- 
sation is  generally  to  stimulate  the  part  concerned  to  in- 
creased growth.  In  some  Orchids  the  ovules  are  not  formed 
in  the  ovary  until  the  stigma  is  pollinated,  and  seem  to  arise 
in  consequence  of  that  process.  The  stimulus  of  fertilisa- 
tion is  still  more  marked.  In  the  Mosses  its  result  is  to 


REPKODUCTION  457 

cause  not  only  the  development  of  the  sporophyte  from 
the  oosphere,  but  a  considerable  additional  growth  of  the 
archegonium,  forming  the  calyptra.  The  same  thing  may  be 
noted  in  those  Rhodophyceae  which  produce  a  bulky  cysto- 
carp.  The  stimulus  is,  however,  most  easily  observed  in 
the  Angiosperms,  where  it  produces  effects  in  several  regions 
of  both  gametophyte  and  sporophyte.  The  oospore  is 
excited  to  growth,  and  after  a  series  of  cell-divisions  becomes 
the  embryo  ;  while  the  definitive  nucleus  of  the  embryo-sac 
similarly  inaugurates  a  series  of  divisions,  ultimately  giving 
rise  to  the  endosperm,  and  other  parts  of  the  ovule  undergo 
modification,  so  that  the  seed  can  shortly  be  recognised. 
Parts  of  the  flower  also  exhibit  renewed  growth  and  further 
development,  the  carpels  especially,  though  not  exclusively, 
showing  an  almost  coincident  enlargement,  which  often 
attains  considerable  dimensions,  so  that  a  bulky  structure 
known  as  the  fruit  is  produced.  The  new  tissue  is  usually 
ordinary  parenchyma,  and  in  most  cases  it  becomes  conspicu- 
ously succulent  and  frequently  strongly  acid.  The  attain- 
ment of  its  maximum  development  is  followed  by  a  process 
technically  known  as  ripening.  This  may  take  one  of  two 
directions  ;  the  tissue  may  become  dry  and  woody,  the  cells 
losing  nearly  all  their  water,  and  their  walls  becoming  con- 
verted into  lignin.  On  the  other  hand  the  succulence  may 
persist  and  even  increase  ;  in  such  cases  the  acidity  fre- 
quently becomes  very  much  diminished  and  a  considerable 
quantity  of  sugar  is  formed.  Other  changes  in  the  cells  lead 
to  the  appearance  of  various  flavouring  matters,  and  often 
of  substances  that  are  aromatic.  Fruits  thus  acquire  special 
characteristics  of  flavour  and  fragrance  which  they  do  not 
possess  while  they  are  young.  The  chemical  changes  which 
give  rise  to  these  peculiarities  are  very  diverse,  and  cannot 
be  said  to  be  fully  understood  at  present. 

We  have  noticed  that  the  asexual  reproductive  cell, 
whether  spore  or  gonidium,  is  generally  found  to  remain 
in  a  state  of  quiescence  for  some  time  after  its  formation. 
The  same  thing  is  seen,  though  not  so  constantly,  in  the 


458  VEGETABLE  PHYSIOLOGY 

case  of  the  zygote.  In  the  Thallophytes  this  resting  period 
is  sometimes  a  long  one  ;  in  the  higher  Cryptogams  it  is 
not  so  noticeable,  and  in  the  Phanerogams  or  Spermo- 
phytes,  where  the  zygote  is  always  developed  inside  the 
sporangium,  it  usually  proceeds  to  active  growth  almost  at 
once.  In  the  latter  plants,  however,  a  resting  period  takes 
place  later,  after  the  seed  is  fully  formed.  The  develop- 
ment of  the  young  sporophyte,  in  fact,  takes  place  in  two 
•stages,  the  one  ending  with  what  may  be  called  the  matura- 
tion of  the  seed,  and  the  other  beginning  with  the  process 
of  germination.  Seeds  when  detached  from  the  parent 
plant  preserve  their  vitality  for  a  variable  length  of  time, 
sometimes  even  for  years,  and  are  capable  of  germinating 
freely  when  exposed  to  favourable  conditions. 

The  germination  of  the  dicotyledonous  seed  occurs  in 
one  of  two  methods.  In  the  first  of  these,  the  cotyledons, 
which  are  thick  and  fleshy,  remain  undergound.  When 
kept  warm  and  moist  the  seed  absorbs  water  and  swells, 
the  testa  bursts,  and  the  radicle,  and  subsequently  the 
plumule,  grow  out  and  elongate  in  opposite  directions.  In 
the  growth  of  the  young  shoot  the  epicotyl,  or  part  between 
the  cotyledons  and  the  first  foliage  leaf  or  leaves,  circumnu- 
tates  and  emerges  in  the  form  of  an  arch,  owing  to  the  greater 
growth  of  one  side.  After  reaching  the  air  the  growth 
changes,  the  greatest  increase  passing  to  the  opposite  side,  so 
that  the  epicotyl  straightens  itself.  During  this  time  it 
subsists  upon  the  nourishment  stored  in  the  cotyledons 
in  the  shape  of  reserve  materials.  We  have  already  dis- 
cussed the  means  whereby  these  digestive  and  nutritive 
changes  are  brought  about,  the  agencies  which  effect  them, 
and  the  various  transformations  which  are  met  with.  As 
the  cotyledons  remain  underground  this  process  is  called 
hypogean  germination.  In  the  other  method — that  of  the 
so-called  epigean  germination — the  cotyledons  sooner  or 
later  rise  above  the  ground  and  become  green,  the  hypocotyl 
behaving  as  does  the  epicotyl  in  the  first  case.  These  are 
frequently,  though  not  always,  albuminous  seeds,  in  which 


KEPRODUCTION  459 

the  nutritive  matter  is  stored  outside  the  embryo.  In 
both  cases  the  root  makes  its  way  into  the  soil  by  virtue 
of  its  geotropism  and  apheliotropism,  aided  by  the  move- 
ment of  circumnutation,  and  by  the  adhesion  of  the  root- 
hairs  to  particles  of  the  soil. 

In  some  Monocotyledons  the  upper  part  of  the  single 
cotyledon  remains  in  the  seed  and  absorbs  the  nutriment 
from  the  endosperm,  while  its  base  elongates  and  thrusts 
the  young  plant  downwards. 

Sometimes  the  usual  alternation  of  sexual  and  asexual 
reproduction  is  interfered  with  by  the  substitution  of  the 
vegetative  method  for  one  of  them.  In  the  phenomenon  of 
apospory,  noticeable  in  some  Ferns,  we  have  small  prothallia 
developed  on  the  back  of  the  leaves  in  the  place  of  spores. 
This  is  a  case  of  the  production  of  a  bud  instead  of  an 
asexual  cell.  Apospory  is  also  known  to  occur  among  the 
Mosses. 

In  the  Ferns,  again,  the  sporophyte  sometimes  arises 
as  a  bud  or  vegetative  outgrowth  upon  the  prothallium,  a 
phenomenon  known  as  apogamy. 

A  curious  phenomenon  is  occasionally  met  with  which  is 
termed  parthenogenesis.  It  occurs  among  the  Fungi,  where, 
as  in  Saprolegnia,  oospheres  are  formed  in  oogonia,  which 
do  not  become  fertilised,  and  yet  have  the  power  of  growing 
out  into  new  plants.  In  some  species  of  Mucor  which  nor- 
mally exhibit  the  fusion  of  particular  hyphae  and  the  admix- 
ture of  their  contents,  or  gametes,  a  variation  of  the  pro- 
cess is  observed  which  comes  under  this  category.  Instead 
of  two  gametangia  meeting,  and  their  contents  fusing, 
to  form  the  zygospore,  these  organs  are  developed  singly  and 
do  not  coalesce.  In  this  case  the  fertile  cell,  which  should 
be  a  zygote,  is  produced  parthenogenetically  in  each,  and  is 
known  as  an  azygospore.  Another  variety  of  partheno- 
genesis, which  resembles  the  apogamy  of  the  Ferns,  occurs 
in  Ccelebogyne,  where  an  embryo  is  produced  in  the  embryo- 
sac,  but  without  pollination  or  fertilisation.  No  sexual  cell 
is  produced,  but  there  occurs  a  vegetative  budding  of  one  or 


460  VEGETABLE  PHYSIOLOGY 

more  of  the  cells  of  the  nucellus  of  the  ovule,  the  buds  grow- 
ing into  the  cavity  of  the  megaspore  and  there  developing 
into  embryos.  This  is  not  quite  the  same  thing  as  the  apo- 
gamy  of  the  Fern,  as  the  new  sporophyte  arises  as  a  bud 
upon  part  of  the  sporangium — that  is,  upon  the  parent  sporo- 
phyte and  not  upon  the  gametophyte.  It  is  really  a  curious 
variation  of  vegetative  propagation. 


INDEX 


Abnormal  methods  of  food  supply, 
189 

Absorption,  of  water  18,  73,  of  food 
materials  132,  of  salts  138,  of  gases 
142,  of  organic  food  189 ;  con- 
ditions of  continuous,  136,  facili- 
tated by  C02  137,  by  acid  sap 

137  ;  strength  of  salts  absorbed, 

138  ;  varied  amounts  of  salts  ab- 
sorbed, 139 

Acclimatisation,  374 

Acer,  92 

Aconitum,  449 

jEthalium,  10 

After-effect  of  stimulation,  417 

Agave,  245 

Aggregation,  400,  414 

Air-chambers,  in  Isoetes,  112 ;  in 
Marsilea,  113  ;  in  submerged 
plants,  114  ;  in  grasses,  115  ;  in 
leaf  of  heath,  119 

Albumen,  444 

Albumin,  167 

Albumoses,  see  Proteoses 

Alburnum,  77 

Alchemilla,  81,  88 

Alcohol,  260,  307 

Aleurone  grains,  241,  242 

Alkaloids,  276 

Alternation  of  generations,  436 

Aluminium,  135,  181 

Amidated  fatty  acids,  276 

Amido-acids,  174 

Ampelopsis,  396 

Amphibious  plants,  342 

Amygdalin,  244,  259 

Anabcena,  203 

Anabolism,  129,  264 

Anaerobic  plants,  309 

Anaesthetics,  action  of.  418 

Analysis,  destructive,  134 

Ananassa,  258 

Anchorage  of  plant,  18 

Anemophilous  flowers,  449 


Antheridia,  434 

Antherozoids,  432  ;  motion  of,  444  ; 

attraction  of,  445 
Anthoceros,  201 
Anthocyan,  273,  328 
Anthoxanthum,  452 
Antipodal  cells,  442,  444 
Apheliotropism,  386 
Aphydrotropism,  400 
Apogamy,  459 
Apogeotropism,  390 
Apospory,  459 
Apostrophe,  371 

Aquatic  plants,  structure  of,  336 
Arabinose,  48 
Archegonia,  434 
Ascending  sap,  71 
Ascogonidia,  429 
Ascomycetes,  434 
Ascospores,  429 
Asexual  reproductive  cells,  426 
Ash  of  plants,  composition  of,  135 
Ash  of  plants,  composition  of,  135, 

176  ;  effect  of  its  constituents  on 

vegetation,  183 
Ash  of  proteins,  166 
Asparagin,     172  ;      as     a     reserve 

material,  244  ;    as  a  product  of 

digestion,  257 
Aspergillus,  256 
Asphyxiation,  306 
Assimilation,  129,  263  ;    (of  carbon 

dioxide,  see  Photosynthesis) 
Auxanometer,  319 
Azobacter,  204 
Azolla,  201,  451 
Azygospore,  459 


BACH,  on  nitrogen  fixation,  172 

Bacteria,  2,  3 

BAEYER,  on  photosynthesis,  166 

Bambusa,  33 

Bark,  24,  232 


461 


462 


VEGETABLE  PHYSIOLOGY 


Bast,  26,  57  ;  function  of,  219,  222, 

224 

Bauhinia,  384 
Begonia,  426 

Bentinckia,    continuity    of    proto- 
plasm in,  16 
Benzol,  174 
Berberis,  392,  407 
Bertholletia,  182 
Betula  lenta,  260 
Biennial  plants,  217 
Bignonia,  386 
Bleeding  (of  stems),  88,  91 
Bloom  (of  fruits),  55 
BOKORNY  on  action  of  chlorophyll 

apparatus,  155 
Botrychium,  191 
BOUILHAC,  157 
Bromelin,  258 
Bromine,  135,  180,  186 
BROWN  and  MORRIS  on  photosyn- 
thesis, 159 
Bryophyllum,  426 
Buckwheat,  179,  186 
Budding,  212,  422 
Buds,  position  of  leaves  in,  320 
Bye-products,  267,  275 


Cabomba,  342 

Calcium,  in  ash ,  135,  141,  180 ; 
mode  of  absorption  of,  183  ;  effect 
of,  on  herbage,  185 

Calcium  pectate,  51 

Calcium,  salts  of,  in  cell- wall,  56, 
279 

Calyptra,  457 

Cambium,  230,  314 

Cane-sugar,  159;  as  reserve  material, 

|    239;  digestion  of ,  256  ;  as  attrac- 

1    tion  for  antherozoids,  402 

Capillarity,  86 

Carbohydrates,  45,  134  ;  formation 
of,  156  ;  theories  of  construction 
of,  156,  159 ;  translocation  of, 
219  ;  resting  and  travelling  forms 
of,  221  ;  storage  of,  234 

Carbon  dioxide,  absorption  of,  120, 
145  ;  exhalation  of,  294  ;  relation 
to  oxygen  absorbed,  291  ;  incap- 
able of  nourishing  protoplasm, 
128 ;  temperature  at  which  decom- 
position by  chloroplast  begins, 
155  ;  optimum  percentage,  162 

Carbon  monoxide  as  a  stage  in 
photosynthesis,  166 


Carica  Papaya,  258 

Castor  oil  plant,  see  Eicinus 

Casuarina,  152,  455 

Cell,  2,  3  ;    various  applications  of 

the  term,  16 
Cell-division,  422 

Cell- wall,  of  protoplast,  2  ;  proper- 
ties of,  44  ;  of  Fungi,  45  ;  theories 
of  its  composition,  48,  49  ;  thick- 
ening of,  49 ;  stratification  of,  50 ; 
formation  of,  269,  425 

Cellulose,  45 ;  properties  of,  45 ; 
varieties  of,  45;  as  reserve 
material,  239,  240 

Centrifugal  force,  influence  on  direc- 
tion of  growth,  391 

Centrospheres,  422 

Ceramium,  16 

Chara,  358 

Chelidonium,  7 

Chemotaxis,  402 

Chitin,  45 

Chlamydomonas,  356,  359 

Chlorine,  180,  186 

Chlorophyll,  146 ;  condition  in 
chloroplasts,  152 ;  solvents  of, 
147 ;  properties  of,  148 ;  spec- 
trum of,  148  ;  analyses  of,  149  ; 
conditions  of  formation  of,  153  ; 
action  of,  dependent  on  light,  154 ; 
relation  to  iron,  183  ;  secretion  of, 
272 

Chlorophyll  apparatus,  146  ;  action 
of,  154  ;  absorption  of  energy  by, 
285,  290 

Chloroplasts,  130,  145,  146  ;  distri- 
bution of,  150  ;  structure  of,  152  ; 
functions  of  the  two  components 
of,  161  ;  effects  of  varying  pres- 
sures of  carbon  dioxide  on,  162  ; 
inhibition  of,  163 

Chlorosis,  154 

Chlorotic  plants,  273 

Chro matin,  8 

Chromosomes,  422 

Chroococcus,  9 

Cilia,  1,  2,  353 

Ciliary  motion,  354 

Circulating  food-stuffs,  218 

Circumnutation,  321  ;  dependent  on 
rhythm,  360 

Cladium,  33 

Cleistogamy,  454 

Cobalt,  181 

Cobalt  chloride  as  test  for  watery 
vapour,  95 

Coelebogyne,  426,  459 


INDEX 


463 


Coenocytes,  structure  of,  10,  11  ; 
skeleton  of,  44  ;  reproduction  of, 
425,  430 

Cold,  injurious  effects  of,  333 

Coleochcete,  436 

Collenchyma,  25,  57 

Commensalism,  201 

Conducting  system,  26 

Conjugation,  434 

Constructive  processes,  anabolic, 
264  ;  katabolic,  266 

Contact,  stimulus  of,  392 

Convolvulus,  396 

Copper,  135,  181 

Cork,  25,  54 

Cortex,  232 

Crassulaceoe,  298,  302 

Crystalloids,  243 

Crystals  of  protein,  243 

Cucumis,  258 

Cucurbita,  92 

Cupuliferce,  204 

Curvature  (of  stimulated  roots),  393 

Cuscuta,  210,  350,  397 

Cuticle,  54  ;  influence  of,  on  tran- 
spiration, 24 

Cuticularisation,  24,  53 

Cutin,  54,  55 

Cutkria,  432 

Cycas,  antherozoids  of,  446,  456 

Cyclamen,  apheliotropism  of  pedun- 
cles of,  386 

Cystocarps,  436 

Cystoliths,  56,  279 

Cytase,  254,  257 

Cytoplasm,  5 

CZAPEK,  409 


DARWIN,  on  localisation  of  sensitive- 
ness, 387,  391  ;  on  tendrils,  394  ; 
on  twining  stems,  396 ;  on  Drosera, 
414 

DARWIN,  F.,  and  PERTZ  on  induced 
rhythm,  404 

Day  and  night,  influence  of  alterna- 
tion of,  379 

Delphinium,  449 

D  jrmatosomes,  49 

Djscending  sap,  223 

D^smids,  358,  388 

Desmodium  gyrans,  362,  379 

Dlageotropism,  390 

Diaheliotropism,  386 

Diastase,  254  ;  function  of,  255  ;  in 
pollen,  454 

Disaster  stage  in  mitosis,  424 


Diatoms,  355 

Dichogamy,  451 

Dlclinism,  452 

Digestion,  129  ;  by  Nepenthes,  196, 
253  ;  by  Drosophyllum,  197  ;  by 
Pinguicula,  197  ;  by  Dioncea, 
200,  253  ;  by  Drosera,  199,  253  ; 
by  Bacteria,  200  ;  of  reserve  ma- 
terials, 248 

Dimorphism,  453 

Dioecious  plants,  452 

Dioncea,  197,  199,  253,  356,  377, 
392,  399,  407,  410,  413,  417 

Dodder,  see  Cuscuta 

Dorsi  ventral  structures,  320 

Double  fertilisation,  447 

Drosera,  199,  253,  356,  392,  399, 
402,  413,  414,  415 

Drosophyllum,  197 

Daramen,  77 


Ectocarpus,  432 

Ectoplasm,  5 

Egg  apparatus,  441 

Elaioplasts,  246,  271 

Elodea,  6,  357 

Embryo,  nutrition  of,*125 

Embryo -sac,  440 

Emulsin,  254,  259 

Endodermis,  75 

Endo-skeleton,  34 

Endosperm,  440,  444 

Energy,  expenditure  of,  in  photosyn- 
thesis, 161,  281  ;  expenditure  of, 
in  transpiration,  281,  287 ;  in 
constructive  processes,  282 ;  in 
growth,  282  ;  in  movement,  283  ; 
in  production  of  heat,  283  ;  of 
light  285  ;  sources  of,  285,  326  ; 
potential  and  kinetic  forms  of, 
288;  liberation  of,  288;  distri- 
bution of,  289 

ENGELMANN,  on  evolution  of  oxygen 
in  different  parts  of  the  spectrum, 
161  ;  on  purple  bacteria,  163 

Entomophilous  flowers,  449 

Environment,  nature  of,  336 

Enzymes,  130,  250  ;  preparation  of, 
262  ;  secretion  of,  268  ;  in  pollen, 
454 

Ephedra,  56 

Epidermis,  characters_of,  53 

Epinasty,  320 

Epiphytes,  201,  346 

Epistrophe,  371 

Equatorial  plate,  422 


464 


VEGETABLE   PHYSIOLOGY 


Equisetacece,  56 

Equisetum,  sclerenchyma  of,  32,  33  ; 

air-spaces  in,   114;'  chloroplasts 

in,  152 
Ereptase,  254 
Eriantkus,  33 

ERLENMEYER  on  photosynthesis,  159 
Erythrophyll,  149 
Erythrozym,  254,  259 
Etiolation,  368 
Etiolin,    153 ;     in    photosynthesis, 

162  ;  "  antecedent  of  chlorophyll, 

273,  368 

Euonymus,  153,  272 
Euphorbia,  237 
EWART  on  inhibition  of  chlorophyll 

apparatus,  163 
Exhaustion,  417 
Exodermis,  25 

Fat,  origin  of,  175 ;  as  reserve 
material,  245  ;  digestion  of,  260  ; 
secretion  of,  271 

Fatigue,  417 

Fermentation,  307,  331 

Fermentative  power  of  protoplasm, 
308 

Ferments  (soluble),  see  Enzymes 

Fern,  skeleton  of,  30 

Fertilisation,  434,  448  ;  stimulating 
effect  of,  456 

Fimbristylis,  33 

Flaccidity,  69 ;  removal  of  by  inject- 
ing water,  96 ;  by  checking  tran- 
spiration, 102 

Flagella,  2 

Flowers,  opening  and  closing  of, 
320,  384 

Fluorine,  181 

Food,  its  nature,  125  ;  formation  of, 
146 ;  conditions  of  continuous 
formation  of,  228 

Food  materials,  relation  to  food, 
127 ;  mode  of  absorption,  132,  134 

Formaldehyde,  as  stage  in  photosyn- 
thesis, 156 ;  in  construction  of 
protein,  172 

Free-cell  formation,  428 

Frost,  action  of,  333 

Fruit,  formation  of,  457 

Fucace.ce,  434 

Fungi,  composition  of  cell-walls  of, 
45  ;  constructive  powers  of,  190  ; 
digestive  powers  of,  200 ;  haus- 
toria  of,  211 

Funkia,  245 

Fusarium,  256 


Gametangia,  432 

Gametes,  430 

Gametophyte,  435 

GARREATJ  on  absorption  of  oxygen 

by  plants,  292 
Gaseous  interchanges,  109 
Gases,  mode  of  absorption  of,  109  ; 

movements  of,  in  plants,  119, 120  ; 

currents  of,  affected  by  external 

conditions,    122 ;    absorption  of, 

142 

Gaultherase,  254,  259 
GAUTIER  on  chlorophyll,  149 
Gelatin,  165 
Gemnae,  426 
Gemmation,  422 
Geotropism,  390 
Germination,    of    pollen,    454 ;     of 

seeds,  458  M 

Ginkgo,  antherozoids  of,  446,  456 
Glands,  251 
Gliadin,  169,  244 
Globoids,  243 
Globulin,  167 
Glochidia,  451 
Glucase,  254,  256 

Glucosides  as  reserve  material,  244 
Gluten,  244 
Glutenin,  169,  244 
Glycogen,  238 
Gonidangia,  428 
Gonidia,  428 
Grafting,  212 

Grape-sugar  as  reserve  material,  239 
Grasses,   construction  of  stem   of, 

115 

Growing-points,  81,  314 
Growth,  216,  310  ;    localisation  of, 

311,  314,  318  ;  conditions  of,  314  ; 

course  of,  315  ;    grand  period  of, 

316,   317;     of   leaf,   318;    daily 

period  of,  319,  361 
Gums,  57 


HARVEY- GIBS  ON  on  photosynthesis, 

161 

Haustoria,  207,  211,  349 
Health,  366,  373 
Heat,  liberation  of,  283  ;  absorption 

of,    327 ;     conduction    of,    331  ; 

regulation  of,  332  ;  resistance  to, 

334 

Heaths,  rolled  leaves  of,  345 
Hedysarum,  362  ;    also  see  Desmo- 

dium 
Helianthus,  96 


INDEX 


465 


Heliotropism,  386,  403,  404 

Hepaticce,  400 

Heterospory,  438 

Heterostylism,  453 

Honey,  450 

Hop,  396 

HOPPE-SEYLER  on  chlorophyll,  149, 
182 

Humus,  72 

Hybridisation,  456 

Hydrocyanic  acid,  173,  259 

Hydrogen  peroxide  in  photosynthe- 
sis, 159,  160 

Hydrophilous  flowers,  449 

Hydrotropism,  400 

Hypnum,  191 

Hyponasty,  320 


Individual,  421 

Insectivorous  plants  ;  Utricularia, 
192  ;  Sarracenia,  194  ;  Nepenthes, 
196  ;  Drosphyllum,  197  ;  Pin- 
guicula,  197  ;  Dioncea,  197,  199  ; 
Drosera,  198 

Intercellular  passages,  36,  37  ;  for- 
mation of,  37,  110  ;  function  of, 
39,  110  ;  watery  vapour  in,  79  ; 
in  Isoetes,  112,  339  ;  in  Marsilea, 
113,  341  ;  relation  to  stomata, 
117  ;  ratio  to  cellular  issue  in 
leaves,  119  ;  composition  of  air 
in,  119,  120 ;  positive  gaseous 
pressure  in,  123  ;  in  aquatic 
plants,  337 

Intercellular  substance,  51 

Internal  pressure  as  a  stimulus,  405 

Intussusception,  50 

Inulase,  254,  256 

Inulin,  238 

Invertase,  254,  256,  454 

Iodine,  135,  180,  186 

Iron,  in  ash  of  plants,  135,  180  ; 
combinations  of  absorbed,  141, 
184  ;  relation  to  chlorophyll,  154  ; 
function  of,  183 

Irritability,  365,  402 

Isoetes,  112,  113,  339,  439 


JUMELLE  on  decomposition  of  car 

bon  dioxide,  155 
Juncus,  33 


Kachree  gourd,  258 
Karyo kinesis,  422 


Katabolism,  129,  264 

Kephir,  202 

Klinostat,  390 

KNIGHT    on    action    of    centrifugal 

force  on  the  direction  of  growth, 

391 


Laccase,  187,  305 

Lactase,  256 

Latent  period  of  response  to  stimu- 
lation, 416 

Lathrcea,  208 

Laticiferous  systems,  226 

Leaves,  sclerenchyma  of,  34  ;  rolled, 
117  ;  irritability  of,  379 

Lecithin,  182,  260 

Leguminosce,  cotyledons  of,  45  ; 
absorption  of  nitrogen  by,  140, 
170,  203  ;  irritability  of  leaves  of, 
379 

Lenticels,  39,  55,  81 

Leucin,  172 

Leucoplasts,  153,  235 

Lichens,  202 

Light,  influence  of,  on  transpiration, 
100 ;  absorption  of,  by  chlorophyll, 
148,  285  ;  necessary  for  formation 
of  chlorophyll,  153  ;  action  in 
photosynthesis,  155,  162  ;  tonic 
influence  of,  367  ;  effect  of,  on 
differentiation  of  leaves,  368  ; 
effect  on  growth,  372  ;  lateral, 
385 

Lignin,  52,  55,  277 

Lipase,  254,  260 

Lithium,  86,  181 

Locomotion,  283,  353,  355 

Lycopodium,  45,  191 

Lythrum,  453 


MACALLUM     on     composition     of 

nucleus,  184 
McNAB  on  measuring  rate  of  ascent 

of  sap,  86 
Magnesium,  in  ash,  135,  141,  180  ; 

mode  of  absorption  of,  183 
Malic  acid,  attraction  for  anthero- 

zoiids,    401 
Maltase,  254 
Malto-dextrin,  255 
Manganese,  135,  180,  186 
Marchantia,  photo-epinasty  of,  371, 

388 

Marsilea,  56,  113 
Massula  of  Azolla,  451 

30 


466 


VEGETABLE   PHYSIOLOGY 


Medullary  rays,  influence  of,  on 
transpiration  current,  87  ;  as  store- 
houses of  reserve  material,  231 

Megaspores,  438 

Melampyrum,  209 

Melibiase,  256 

Melizitase,  256 

Mesocarpece,  431 

Mesocarpus,  365,  371 

Metabolism,  6,  129,  264 

Metapectic  acid,  47 

Metapectine,  47 

Meta- proteins,  168 

Methane,  174 

Micellae  of  cell- wall.  48 

Microbes,  307 

Micrococcus  Urece,  261 

Microsomata,  269 

Microspores,  438 

Middle  lamella,  51 

Mimosa,  67,  363,  365,  377,  379,  393, 
397 

Mimulus,  392 

Mistletoe,  205,  350 

Mitosis,  422 

MIYOSHI  on  chemotaxis,  402 

MonoblepTiaris,  433 

Monoecious  plants,  452 

Monotropa,  204 

Moorland  plants,  345 

Movements,  of  protoplasm,  6,  357  ; 
for  capture,  197,  199  ;  in  multi- 
cellular  organs,  358 

Mucilage,  56,  240 

Mucor,  459 

MULLIKIN  BROWN  and  FRENCH  on 
formaldehyde,  157 

Mycoderma  aceti,  262 

Mycorhiza,  204 

Myriophyllum,  123 

Myronate  of  potash,  see  Sinigrin 

Myrosin,  254,  259 

Myxomycetes,  10,  354,  400,  401,  402 


NAEGELI,  theory  of  composition  of 
cell-wall,  48 

Nectar,  278 

Nectary,  68,  450 

Negative  pressure,  in  wood-vessels, 
94,  97  ;  influence  of,  on  move- 
ments of  gases,  122 

Neottia,  191 

Nepenthes,  196,  253 

Nervous  mechanisms,  406 

Nickel,  181    . 

Nicotiana,  380 


Nitella,  6,  358 

Nitrates,  135,  171 

Nitrification,  139,  163 

Nitrogen,  absorption  of,  120,  139  ; 

in  proteins,  166  ;    fixation  of,  by 

leguminous  plants,  140,  170,  203, 

by  Algae,  171 
Nostoc,  12,  201 
Nuclear  spindle,  269,  422 
Nuclein,  8,  180,  182 
Nucleolus,  8 
Nucleoplasm,  8 
Nucleus,   5 ;     position    in    cell,    7 ; 

structure    of,  8 ;    definitive,  442 ; 

generative,  446 
Nutation,  321 
Nyctitropic   movements,   331,   380, 

382 
Nymphcea,  115,  339 

Ocelli,  410 

(Edogonium,  436 

Oil,  see  Fat, 

Oncidium,  245 

Oogonia,  432 

Oospheres,  432 

Oospore,  434 

Optically  active  compounds,  158 

Opuntia,  298,  302 

Ornithogalum,  245 

Orobanchacece,  350 

Orobanche,  208 

Osmosis,  60  ;  in  adult  cells,  65  ;  from 
veins  of  leaf,  79,  105  ;  influence 
of,  on  transpiration  current,  106, 
108 

Osmotic  pressure,  61 

Ouvirandra,  339 

Ovule,  440 

Oxalic  acid,  302 

Oxalidacece,  379 

Oxalis  acetosella,  370 

Oxidases,  261,  265,  305 

Oxidation,  305 

Oxygen,  mode  of  absorption  by 
plants,  15,  35,  120  ;  influence  of, 
on  action  of  roots,  90  ;  proof  of 
absorption  of,  292 ;  relation  of 
absorption  of,  to  exhalation  of 
carbon  dioxide,  297  ;  variation  in 
amount  of,  as  affecting  respiration, 
306 

Paeonia,  45. 
Pangium,  173 
Papai'n,  258 


INDEX 


467 


Paraheliotropism,  370 

Parapectine,  47 

Parasites,  127,  347 

Parasitism,  191,  196,  205 

Paratonic  influence  of  light,  367 

Parnassia,  452 

Parthenogenesis,  459 

Passiflora,  394,  407 

Pectase,  254,  257 

Pectic  acid,  47  ;  relation  to  middle 
lamella,  51 

Pectine,  46 

Pectose,  46 

Pediastrum,  12 

Pelvetia,  22 

Pennisetum,  32,  33 

Peptase,  254,  257 

Peptone,  168 

Perception,  415 

Periodicity  of  root-pressure,  92,  364 

Perisperm,  444 

PFEFFER,  on  water-culture,  172  ;  on 
localisation  of  sensitiveness,  391 

Phajus,  237 

Phalaris,  386,  407,  409 

Phosphorus,  in  proteins,  135,  180  ; 
mode  of  absorption  of,  141  ;  asso- 
ciated with  nucleus,  182 

Photo-epinasty,   371 

Photosynthesis,  155 ;  theories  of, 
156  ;  not  carried  out  by  fungi, 
164 

Phototaxis,  388 

Phototonus,  366 

Phytol,  150 

Pinguicula,   197 

Pitcher-plants,  see  Insectivorous 
plants 

Pitchers,  195 

Planogametes,  431 

Plantago,  452 

Plasmodium  of  Myxomycetes,  4,  10, 
354 

Plasmolysis,  64 

Plastids,  5 

Polar  nuclei,  441 

Pollen- grain,  germination  of,  454 

Pollen-tube,  442,  446 

Pollination,  448  ;  methods  of,  449 

Polyembryony,  447 

Polygamous  plants,  452 

Polygonum,   photo-epinasty  of,  371 

Polymerisation  of  aldehydes,  158 

Polytrichum,  31 

Porlieria,  400 

Positive  pressure  of  gases  in  inter- 
cellular reservoirs,  121 


Potamogeton,  36 

Potassium,  in  ash,  135,  141,  180  ; 
condition  of,  in  soil,  141  ;  mode  of 
absorption  of,  183  ;  function  of, 
185 

Potometer,  103 

Predominant  form  of  plants  in 
different  groups,  437 

Prepotency,  453 

Primula,  275 

Procarpium,  433 

Protandry,  451 

Proteins,  165  ;  composition  of,  166  ; 
crystals  of,  166  ;  reactions  of,  166  ; 
coagulated,  167  ;  construction  of, 
169  ;  translation  of,  221,  224  ; 
storage  of  222 

Proteoclastic  enzymes,  254,  257 

Proteoses,  168 

Protococcus,  9 

Protogyny,  452 

Protoplasm,  composition  of,  6; 
movements  of  6,  349  ;  properties 
of,  12 ;  continuity  of,  through 
cell- walls,  15,  413  ;  regulation  of 
osmosis  by,  65,  107  ;  relation  of, 
to  respiration,  299 ;  nutritive 
substances  for,  127  ;  fermenta- 
tive activity  of,  250,  261,  308  ; 
permeabilitv  of,  by  water,  359, 
412 

Protoplasts,  structure  of,  2 ;  arrange- 
ment of,  in  multicellular  plants,  3 

Prunus,  92,  259 

Prussic  acid,  245,  259 

Pseudomonas,  204 

Pseudopodia  of  Myxomycetes.  4, 
354 

Pulvinus,  67,  362,  382,  399 

Pythium,  433 


Radiation,  284,  330 

Raffinose,  239 

Rafllesia,  209 

Raphides,  279 

Rectipetality,  324 

Reductases,  261,  266 

Reflex  action,  415 

Rennet,  254,  259 

Reproduction,     420 ;      relation    to 

growth,    421  ;     vegetative.    426 ; 

asexual,  427 ;  sexual,  430 
Reserve  materials,  227 
Reservoirs,  of  air,  113;  of  food,  130, 

228 
Resin,  275 


468 


VEGETABLE   PHYSIOLOGY 


Respiration,  291  ;  gaseous  inter- 
changes of,  291 ;  masked  by  photo- 
synthesis, 292  ;  loss  of  weight  in- 
volved in,  295  ;  intensity  of,  297  ; 
nature  of  the  process  of,  299  ; 
relation  of,  to  metabolism,  300 ; 
influence  of  external  conditions 
on,  302  ;  of  seeds,  303  ;  a  source 
of  energy,  304  ;  intramolecular  or 
anaerobic,  306 

Respiratory  quotient,  297 

Rhamnase,  254,  259 

Rhamnus,  259 

Rhodophyceaz,  434 

Rhythm,  of  root-pressure,  88 ; 
nature  of,  360  ;  affected  by  stimu- 
lation, 403  ;  induced,  404 

Ricinus,  182,  243 

Ripening,  457 

Robinia,  67 

Root-hairs,  73  ;    action  of,  74 

Root-parasites,  206,  350 

Root-pressure,  76,  86,  87  ;  rhythm 
of,  88,  91  ;  method  of  measuring, 
89,  91  ;  amount  of,  90 ;  period- 
icity of,  92 

Rush,  construction  of  stem  of, 
116 


Saccharomyces,  see  Yeast 

SACHS,  on  theory  of  transport  of 
water  in  wood,  84 

Salvinia,  113  ;  germination  of  mega- 
spore  of,  439 

Santalaceoe,  206,  350 

Saprolegnia,  402,  428,  459 

Saprophytes,  127,  191,  348 

Sarracenia,  194 

SAUSSUBE,  DE,  on  Opuntia,  302 

Saxifraga,  253,  344 

Saxifragaceoe,  298 

Scirpus,  32,  33 

Sclerenchyma,  25,  29,  30,  34 

Scrophularia,  452 

Scrophulariacece,  206,  350 

Scutellum,  epithelium  of,  252, 
255 

Scyphanfhus,  396 

Secretion,  267 

Seed,  440  ;  formation  of,  441  ;  struc- 
ture of,  444 ;  germination  of, 
458 

Selaginella,  prothallium  of,  439 

Selective  power,  181,  187 

Self-sterility,  453 

Sense  organs,  411 


Sensitiveness,  365 ;  variations  of, 
386;  localisation  of,  386,  392, 
411  ;  delicacy  of,  394,  418  ; 
maintenance  of,  417 

Sex,  differentiation  of,  432 

Sexual  cells,  430 

Sieve-tubes,  175,  224 

Silica,  56,  138,  279 

Silicates,  142,  186 

Silicic  acid,  142 

Silicon,  135,  141,  180,  185 

Sinigrin,  245,  259 

Skeleton,  of  plastid,  2,  42  ;  of  plant, 
34,  42 

Sleep  movements,  see  Nyctitropic 
movements 

Slime  fungi,  see  Myxomycetes 

Sodium,  135,  141,  180,  185 

Soil,  composition  of,  72 

Sorus,  451 

Spermatium,  445 

Spermatozoids,  432 

Sphagnum,  22 

Spirogyra,  150,  157,  420,  426 

Splachnum,  191 

Sporangia,  428 

Spores,  428,  435 

Sporophyte,  435 

STAHL  on  method  of  detecting  escape 
of  watery  vapour,  95 

Starch,  appearance  of,  during  photo- 
synthesis, 158  ;  probable  cause  of 
appearance  of,  in  leaf,  160 ;  forma- 
tion of,  as  evidence  of  activity  of 
chlorophyll  apparatus,  161 ;  depo- 
sition of,  in  chloroplasts,  219,  234; 
removal  of,  from  leaves,  230 ; 
storage  of,  234  ;  deposition  of,  by 
leucoplasts,  235  ;  by  protoplasm, 
237  ;  digestion  of,  254  ;  secretion 
of,  270 

Statoliths,  392 

Stele  of  root,  75 

Stereome,  34 

Stimulation,  375  ;  nature  of,  377  ; 
purposeful  nature  of  responses  to, 
377, 415  ;  localisation  of,  387,  395  ; 
after-effects  of,  407  ;  perception 
of,  415  ;  latent  period  of,  416 

Stimulus,  nature  of,  375  ;  instances 
of  rhythmic,  379,  381  ;  of  lateral 
light,  385  ;  of  gravitation,  388  ;  of 
contact,  392  ;  of  moisture,  400  ; 
of  oxygen,  401  ;  chemical,  400  ; 
relationship  of  response  to,  415  ; 
of  fertilisation  and  pollination, 
456 


INDEX 


469 


Stomata,  37,  80,  97  ;  number  of, 
in  different  plants,  99 ;  mode  of 
action  of,  99  ;  regulating  influence 
of,  on  transpiration,  100  ;  relation 
to  gaseous  interchanges,  117,  123 

Storage  of  food,  217,  230 

STRASBUBGEB,  on  composition  of 
cell- walls,  49  ;  on  evaporation  of 
water  from  cell-halls,  105 

Stratification  of  cell- wall,  50 

Style,  reserve  materials  in,  233 

Stylogonidia,  429 

Stylospores,  429 

Suberin,  277 

Sugar,  formation  of,  158,  160  ;  fer- 
mentation of,  307 

Sulphur,  in  proteins,  166 ;  in 
protoplasm,  182  ;  mode  of  absorp- 
tion, of,  141,  182 

Sundew,  see  Drosera 

Sunlight,  influence  of,  on  transpira- 
tion, 100 

Surplus  food,  construction  of,   215 

Symbiosis,  191,  201 

Synergidae,  442 

Tegumentary  system,  24 

Telegraph  plant,  see  Desmodium 

Temperature,  influence  of,  on  root- 
pressure,  90 ;  on  transpiration, 
101 ;  on  respiration,  302 ;  range  of, 
for  different  functions,  325  ; 
fluctuations  of,  326 

Tendrils,  394 

Tensions,  in  hollow  stems,  34  ;  in 
growing  organs,  322 

Testa  of  seed,  444 

Thalictrum,  449 

Theca,  436 

Thermotonus,  366 

Thesium,  206 

TIMIRIAZEFF  on  relative  values  of 
the  rays  of  the  spectrum  in  photo- 
synthesis, 162 

Tone,  366,  371,  373 

Torsion  of  stems,  396 

Tradescantia,  7,  358 

Translocation,  of  food,  213  ;  direc- 
tion of,  222  ;  path  of,  225 

Transpiration,  80,  86,  93  ;  methods 
of  demonstrating,  93  ;  amount 
of,  96 ;  effect  of  excessive, 
96 ;  influence  of  external  con- 
ditions on,  100  ;  suction  of,  105  ; 
functions  of,  330 ;  relation  to 
etiolation,  368 

Transpiration  current,  83,  85 


Trees,  conditions  of  life  of,  28 

TREBOUX,  157 

Trehalase,  256 

TRETJB  on  construction  of  protein 

in  Pangium,  173 
Trichogyne,  432 
Tropceoum,  88,  159 
Tryptase,  254,  257 
Turgescence,    67  ;      importance   of, 

69 ;      methods    of    restoring,    in 

flaccid    tissue,    96 ;     affected    by 

shaking  branches,  102 
Turgor,  358  ;    also  see  Turgescence 
Twigs,  reserve  materials  in,  233 
Twining  organs,  396 
Typha,  33 
Ty  rosin,  172 
Tyiosinase,  305 

Ulothrix,  1,  421,  427,  431,  432 

UNGER  on  relative  volumes  of  air 
and  cellular  tissue  in  leaves,  119 

Unicellular  plants,  2 

Urea,  141 
•Urease,  261 

USHER  AND  PRIESTLEY,  on  photo- 
synthesis, 160 

Vacuole,  4  ;  formation  of  13,  62  ; 
function  of,  14 ;  occasional  ab- 
sence of,  59  ;  pulsating,  356,  359 

Vallisneria,  6,  358 

Vascular  bundles,  26,  31 

Vaucheria,  430 

Vegetable  acids,  277 

Vegetative  propagation,  426 

Veins  of  leaf,  29,  77 

Velamen,  347 

Venus's  fly-trap,  see  Dioncea 

Vicia,  409 

Victoria  regia,  340 

VINES  on  photosynthesis,  159  :  on 
ereptase,  259 

Viola,  454 

Vitis,  92 

Volvox,  10,  11 

Water,  importance  of,  to  proto- 
plasm, 13,  14 ;  to  plant  in  general, 
14;  absorption  of,  by  aquatic 
plants,  27  ;  function  of,  in  cell, 
59  ;  circulation  of,  in  plant,  59  ; 
exhalation  of  vapour  of,  66,  299  ; 
hygroscopic,  73  ;  absorption  by 
root-hairs,  73  ;  transport  in  root, 
74 ;  in  stem,  77 ;  excretion  of, 
81 ;  mode  of  transport  in  wood, 


470 


VEGETABLE   PHYSIOLOGY 


84  ;  exudation  of,  under  pressure, 
88;  exudation  affected  by  salts, 
91 

Water-culture,  132,  177 

Water-glands,  344 

Wax  in  cell- walls,  55 

WIESNEB  on  composition  of  cell- 
wall,  49  ;  on  heliotropism,  407 

WILSTATTER  on  chlorophyll,  149 

Withering,  96 

Wood,  26 


Xanthophyll,  149,  277 
Xerophytes,  343 


Yeast,  2,  307,  421,  429 


Zein,  169 
Zinc,  135,  181 
Zirconium,  181 
Zoocoenocyte,  430 
Zoogonidia,  1,  428 
Zoophilous  flowers,  449 
Zoospores,  1,  428 
Zygnema,  150,  431 
Zygospore,  431,  434 
Zygote,  434 
Zymase,  260,  307 
Zymogen,  251 


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